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/panic.go

@@ -0,0 +1,33 @@
+package transform
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// ReplacePanicsWithTrap replaces each call to panic (or similar functions) with
+// calls to llvm.trap, to reduce code size. This is the -panic=trap command-line
+// option.
+func ReplacePanicsWithTrap(mod llvm.Module) {
+	ctx := mod.Context()
+	builder := ctx.NewBuilder()
+	defer builder.Dispose()
+
+	trap := mod.NamedFunction("llvm.trap")
+	if trap.IsNil() {
+		trapType := llvm.FunctionType(ctx.VoidType(), nil, false)
+		trap = llvm.AddFunction(mod, "llvm.trap", trapType)
+	}
+	for _, name := range []string{"runtime._panic", "runtime.runtimePanic"} {
+		fn := mod.NamedFunction(name)
+		if fn.IsNil() {
+			continue
+		}
+		for _, use := range getUses(fn) {
+			if use.IsACallInst().IsNil() || use.CalledValue() != fn {
+				panic("expected use of a panic function to be a call")
+			}
+			builder.SetInsertPointBefore(use)
+			builder.CreateCall(trap, nil, "")
+		}
+	}
+}