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_test.go

@@ -0,0 +1,15 @@
+package transform
+
+import (
+	"testing"
+
+	"tinygo.org/x/go-llvm"
+)
+
+func TestOptimizeStringToBytes(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/stringtobytes", func(mod llvm.Module) {
+		// Run optimization pass.
+		OptimizeStringToBytes(mod)
+	})
+}