compiler: move OptimizeStringToBytes to transform package

Unfortunately, while doing this I found that it doesn't actually apply
in any real-world programs (tested with `make smoketest`), apparently
because nil pointer checking messes with the functionattrs pass. I hope
to fix that after moving to LLVM 9, which has an optimization that makes
nil pointer checking easier to implement.

transform/stringtobytes.go

@@ -0,0 +1,57 @@
+package transform
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// OptimizeStringToBytes transforms runtime.stringToBytes(...) calls into const
+// []byte slices whenever possible. This optimizes the following pattern:
+//
+//     w.Write([]byte("foo"))
+//
+// where Write does not store to the slice.
+func OptimizeStringToBytes(mod llvm.Module) {
+	stringToBytes := mod.NamedFunction("runtime.stringToBytes")
+	if stringToBytes.IsNil() {
+		// nothing to optimize
+		return
+	}
+
+	for _, call := range getUses(stringToBytes) {
+		strptr := call.Operand(0)
+		strlen := call.Operand(1)
+
+		// strptr is always constant because strings are always constant.
+
+		convertedAllUses := true
+		for _, use := range getUses(call) {
+			if use.IsAExtractValueInst().IsNil() {
+				// Expected an extractvalue, but this is something else.
+				convertedAllUses = false
+				continue
+			}
+			switch use.Type().TypeKind() {
+			case llvm.IntegerTypeKind:
+				// A length (len or cap). Propagate the length value.
+				use.ReplaceAllUsesWith(strlen)
+				use.EraseFromParentAsInstruction()
+			case llvm.PointerTypeKind:
+				// The string pointer itself.
+				if !isReadOnly(use) {
+					convertedAllUses = false
+					continue
+				}
+				use.ReplaceAllUsesWith(strptr)
+				use.EraseFromParentAsInstruction()
+			default:
+				// should not happen
+				panic("unknown return type of runtime.stringToBytes: " + use.Type().String())
+			}
+		}
+		if convertedAllUses {
+			// Call to runtime.stringToBytes can be eliminated: both the input
+			// and the output is constant.
+			call.EraseFromParentAsInstruction()
+		}
+	}
+}