compiler: insert basic blocks at an appropriate location

For example, this commit moves the 'throw' branch of an assertion (nil
check, slice index check, etc) to the end of the function while
inserting the "continue" branch right after the insert location. This
makes the resulting IR easier to follow.

For some reason, this also reduces code size a bit on average. The
TinyGo smoke tests saw a reduction of 0.22%, mainly from WebAssembly.
The drivers repo saw little average change in code size (-0.01%).

This commit also adds a few compiler tests for the defer keyword.

compiler/testdata/go1.17.ll

@@ -36,13 +36,13 @@ entry:
   %0 = icmp ult i32 %s.len, 4
   br i1 %0, label %slicetoarray.throw, label %slicetoarray.next
 
-slicetoarray.throw:                               ; preds = %entry
-  call void @runtime.sliceToArrayPointerPanic(i8* undef) #0
-  unreachable
-
 slicetoarray.next:                                ; preds = %entry
   %1 = bitcast i32* %s.data to [4 x i32]*
   ret [4 x i32]* %1
+
+slicetoarray.throw:                               ; preds = %entry
+  call void @runtime.sliceToArrayPointerPanic(i8* undef) #0
+  unreachable
 }
 
 declare void @runtime.sliceToArrayPointerPanic(i8*)
@@ -54,12 +54,12 @@ entry:
   call void @runtime.trackPointer(i8* nonnull %makeslice, i8* undef) #0
   br i1 false, label %slicetoarray.throw, label %slicetoarray.next
 
-slicetoarray.throw:                               ; preds = %entry
-  unreachable
-
 slicetoarray.next:                                ; preds = %entry
   %0 = bitcast i8* %makeslice to [4 x i32]*
   ret [4 x i32]* %0
+
+slicetoarray.throw:                               ; preds = %entry
+  unreachable
 }
 
 ; Function Attrs: nounwind
@@ -72,10 +72,6 @@ entry:
   %4 = or i1 %3, %0
   br i1 %4, label %unsafe.Slice.throw, label %unsafe.Slice.next
 
-unsafe.Slice.throw:                               ; preds = %entry
-  call void @runtime.unsafeSlicePanic(i8* undef) #0
-  unreachable
-
 unsafe.Slice.next:                                ; preds = %entry
   %5 = insertvalue { i32*, i32, i32 } undef, i32* %ptr, 0
   %6 = insertvalue { i32*, i32, i32 } %5, i32 %len, 1
@@ -83,6 +79,10 @@ unsafe.Slice.next:                                ; preds = %entry
   %8 = bitcast i32* %ptr to i8*
   call void @runtime.trackPointer(i8* %8, i8* undef) #0
   ret { i32*, i32, i32 } %7
+
+unsafe.Slice.throw:                               ; preds = %entry
+  call void @runtime.unsafeSlicePanic(i8* undef) #0
+  unreachable
 }
 
 declare void @runtime.unsafeSlicePanic(i8*)
@@ -95,10 +95,6 @@ entry:
   %2 = and i1 %0, %1
   br i1 %2, label %unsafe.Slice.throw, label %unsafe.Slice.next
 
-unsafe.Slice.throw:                               ; preds = %entry
-  call void @runtime.unsafeSlicePanic(i8* undef) #0
-  unreachable
-
 unsafe.Slice.next:                                ; preds = %entry
   %3 = zext i16 %len to i32
   %4 = insertvalue { i8*, i32, i32 } undef, i8* %ptr, 0
@@ -106,6 +102,10 @@ unsafe.Slice.next:                                ; preds = %entry
   %6 = insertvalue { i8*, i32, i32 } %5, i32 %3, 2
   call void @runtime.trackPointer(i8* %ptr, i8* undef) #0
   ret { i8*, i32, i32 } %6
+
+unsafe.Slice.throw:                               ; preds = %entry
+  call void @runtime.unsafeSlicePanic(i8* undef) #0
+  unreachable
 }
 
 ; Function Attrs: nounwind
@@ -118,10 +118,6 @@ entry:
   %4 = or i1 %3, %0
   br i1 %4, label %unsafe.Slice.throw, label %unsafe.Slice.next
 
-unsafe.Slice.throw:                               ; preds = %entry
-  call void @runtime.unsafeSlicePanic(i8* undef) #0
-  unreachable
-
 unsafe.Slice.next:                                ; preds = %entry
   %5 = trunc i64 %len to i32
   %6 = insertvalue { i32*, i32, i32 } undef, i32* %ptr, 0
@@ -130,6 +126,10 @@ unsafe.Slice.next:                                ; preds = %entry
   %9 = bitcast i32* %ptr to i8*
   call void @runtime.trackPointer(i8* %9, i8* undef) #0
   ret { i32*, i32, i32 } %8
+
+unsafe.Slice.throw:                               ; preds = %entry
+  call void @runtime.unsafeSlicePanic(i8* undef) #0
+  unreachable
 }
 
 ; Function Attrs: nounwind
@@ -142,10 +142,6 @@ entry:
   %4 = or i1 %3, %0
   br i1 %4, label %unsafe.Slice.throw, label %unsafe.Slice.next
 
-unsafe.Slice.throw:                               ; preds = %entry
-  call void @runtime.unsafeSlicePanic(i8* undef) #0
-  unreachable
-
 unsafe.Slice.next:                                ; preds = %entry
   %5 = trunc i64 %len to i32
   %6 = insertvalue { i32*, i32, i32 } undef, i32* %ptr, 0
@@ -154,6 +150,10 @@ unsafe.Slice.next:                                ; preds = %entry
   %9 = bitcast i32* %ptr to i8*
   call void @runtime.trackPointer(i8* %9, i8* undef) #0
   ret { i32*, i32, i32 } %8
+
+unsafe.Slice.throw:                               ; preds = %entry
+  call void @runtime.unsafeSlicePanic(i8* undef) #0
+  unreachable
 }
 
 attributes #0 = { nounwind }