compiler: move ReplacePanicsWithTrap pass to transforms

This moves the transformation pass to the right location, and adds tests
to see that it actually works correctly.

transform/testdata/panic.ll

@@ -0,0 +1,22 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@"runtime.lookupPanic$string" = constant [18 x i8] c"index out of range"
+
+declare void @runtime.runtimePanic(i8*, i32)
+
+declare void @runtime._panic(i32, i8*)
+
+define void @runtime.lookupPanic() {
+  call void @runtime.runtimePanic(i8* getelementptr inbounds ([18 x i8], [18 x i8]* @"runtime.lookupPanic$string", i64 0, i64 0), i32 18)
+  ret void
+}
+
+; This is equivalent to the following code:
+;     func someFunc(x interface{}) {
+;         panic(x)
+;     }
+define void @someFunc(i32 %typecode, i8* %value) {
+  call void @runtime._panic(i32 %typecode, i8* %value)
+  unreachable
+}

transform/testdata/panic.out.ll

@@ -0,0 +1,25 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@"runtime.lookupPanic$string" = constant [18 x i8] c"index out of range"
+
+declare void @runtime.runtimePanic(i8*, i32)
+
+declare void @runtime._panic(i32, i8*)
+
+define void @runtime.lookupPanic() {
+  call void @llvm.trap()
+  call void @runtime.runtimePanic(i8* getelementptr inbounds ([18 x i8], [18 x i8]* @"runtime.lookupPanic$string", i64 0, i64 0), i32 18)
+  ret void
+}
+
+define void @someFunc(i32 %typecode, i8* %value) {
+  call void @llvm.trap()
+  call void @runtime._panic(i32 %typecode, i8* %value)
+  unreachable
+}
+
+; Function Attrs: cold noreturn nounwind
+declare void @llvm.trap() #0
+
+attributes #0 = { cold noreturn nounwind }