transform: optimize string comparisons against ""

This optimizes a common pattern like:

    if s != "" {
        ...
    }

to:

    if len(s) != 0 {
        ...
    }

This avoids a runtime call and thus produces slightly better code.

transform/rtcalls_test.go

@@ -0,0 +1,23 @@
+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)
+	})
+}
+
+func TestOptimizeStringEqual(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/stringequal", func(mod llvm.Module) {
+		// Run optimization pass.
+		OptimizeStringEqual(mod)
+	})
+}