pass: remove redundant jumps and dangling labels (#81)

In jump-table-like constructs, the natural way of writing the code can sometimes produce redundant jumps or labels. Therefore some basic cleanup steps have been proposed. This diff adds two transforms:

1. Remove unconditional jumps to a label immediately following.
2. Remove labels with no references at all.

Fixes #75

tests/labels/asm.go

@@ -0,0 +1,28 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+	. "github.com/mmcloughlin/avo/reg"
+)
+
+func main() {
+	TEXT("Labels", NOSPLIT, "func() uint64")
+	XORQ(RAX, RAX)
+	INCQ(RAX)
+	Label("neverused")
+	INCQ(RAX)
+	INCQ(RAX)
+	INCQ(RAX)
+	INCQ(RAX)
+	JMP(LabelRef("next"))
+	Label("next")
+	INCQ(RAX)
+	INCQ(RAX)
+	Store(RAX, ReturnIndex(0))
+	RET()
+
+	Generate()
+}

tests/labels/doc.go

@@ -0,0 +1,2 @@
+// Package labels tests for cleanup of redundant labels.
+package labels

tests/labels/labels.s

@@ -0,0 +1,16 @@
+// Code generated by command: go run asm.go -out labels.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Labels() uint64
+TEXT ·Labels(SB), NOSPLIT, $0-8
+	XORQ AX, AX
+	INCQ AX
+	INCQ AX
+	INCQ AX
+	INCQ AX
+	INCQ AX
+	INCQ AX
+	INCQ AX
+	MOVQ AX, ret+0(FP)
+	RET

tests/labels/labels_test.go

@@ -0,0 +1,15 @@
+package labels
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out labels.s -stubs stub.go
+
+func TestLabels(t *testing.T) {
+	expect := func() uint64 { return 7 }
+	if err := quick.CheckEqual(Labels, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}

tests/labels/stub.go

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