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

tests/fixedbugs/issue76/issue76_test.go

@@ -0,0 +1,15 @@
+package issue76
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out issue76.s -stubs stub.go
+
+func TestIssue76(t *testing.T) {
+	expect := func(x, y uint64) uint64 { return x + y }
+	if err := quick.CheckEqual(Issue76, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}