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/string.ll

@@ -41,14 +41,14 @@ entry:
   %.not = icmp ult i32 %index, %s.len
   br i1 %.not, label %lookup.next, label %lookup.throw
 
-lookup.throw:                                     ; preds = %entry
-  call void @runtime.lookupPanic(i8* undef) #0
-  unreachable
-
 lookup.next:                                      ; preds = %entry
   %0 = getelementptr inbounds i8, i8* %s.data, i32 %index
   %1 = load i8, i8* %0, align 1
   ret i8 %1
+
+lookup.throw:                                     ; preds = %entry
+  call void @runtime.lookupPanic(i8* undef) #0
+  unreachable
 }
 
 declare void @runtime.lookupPanic(i8*)
@@ -86,14 +86,14 @@ entry:
   %.not = icmp ult i32 %0, %s.len
   br i1 %.not, label %lookup.next, label %lookup.throw
 
-lookup.throw:                                     ; preds = %entry
-  call void @runtime.lookupPanic(i8* undef) #0
-  unreachable
-
 lookup.next:                                      ; preds = %entry
   %1 = getelementptr inbounds i8, i8* %s.data, i32 %0
   %2 = load i8, i8* %1, align 1
   ret i8 %2
+
+lookup.throw:                                     ; preds = %entry
+  call void @runtime.lookupPanic(i8* undef) #0
+  unreachable
 }
 
 attributes #0 = { nounwind }