pass: add PruneSelfMoves cleanup pass (#80)

In some cases natural use of abstraction in avo programs can lead to redundant move instructions, specifically self-moves such as MOVQ AX, AX. This does not produce incorrect code but it is incorrect and inelegant.

This diff introduces a PruneSelfMoves pass that removes such unnecessary instructions.

Closes #76

pass/util_test.go

@@ -0,0 +1,36 @@
+package pass_test
+
+import (
+	"testing"
+
+	"github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/internal/test"
+	"github.com/mmcloughlin/avo/ir"
+	"github.com/mmcloughlin/avo/pass"
+)
+
+// BuildFunction is a helper to compile a build context containing a single
+// function and (optionally) apply a list of FunctionPasses to it.
+func BuildFunction(t *testing.T, ctx *build.Context, passes ...pass.FunctionPass) *ir.Function {
+	t.Helper()
+
+	f, err := ctx.Result()
+	if err != nil {
+		build.LogError(test.Logger(t), err, 0)
+		t.FailNow()
+	}
+
+	fns := f.Functions()
+	if len(fns) != 1 {
+		t.Fatalf("expect 1 function")
+	}
+	fn := fns[0]
+
+	for _, p := range passes {
+		if err := p(fn); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	return fn
+}