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()
+}