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/asm.go

@@ -0,0 +1,29 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/reg"
+)
+
+func main() {
+	TEXT("Issue76", NOSPLIT, "func(x, y uint64) uint64")
+	x := Load(Param("x"), GP64())
+	y := Load(Param("y"), GP64())
+	s := add(x, y)
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}
+
+// add generates code to add x and y. The intent here is to demonstrate how a
+// natural subroutine in avo typically requires temporary registers, which in
+// turn can be "optimized out" by the register allocator and result in redundant
+// self-moves.
+func add(x, y Register) Register {
+	s := GP64()
+	MOVQ(x, s) // likely to become a self move
+	ADDQ(y, s)
+	return s
+}

tests/fixedbugs/issue76/doc.go

@@ -0,0 +1,4 @@
+// Package issue76 deliberately produces redundant MOV instructions.
+//
+// The intent is to confirm redundant self-move instructions are correctly removed.
+package issue76

tests/fixedbugs/issue76/issue76.s

@@ -0,0 +1,11 @@
+// Code generated by command: go run asm.go -out issue76.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Issue76(x uint64, y uint64) uint64
+TEXT ·Issue76(SB), NOSPLIT, $0-24
+	MOVQ x+0(FP), AX
+	MOVQ y+8(FP), CX
+	ADDQ CX, AX
+	MOVQ AX, ret+16(FP)
+	RET

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)
+	}
+}

tests/fixedbugs/issue76/stub.go

@@ -0,0 +1,5 @@
+// Code generated by command: go run asm.go -out issue76.s -stubs stub.go. DO NOT EDIT.
+
+package issue76
+
+func Issue76(x uint64, y uint64) uint64