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/fmt/asm.go

@@ -4,6 +4,7 @@ package main
 
 import (
 	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
 	. "github.com/mmcloughlin/avo/reg"
 )
 
@@ -19,6 +20,7 @@ func main() {
 	Label("label")
 	Comment("Comment after label.")
 	ADDQ(R8, R8)
+	JMP(LabelRef("label"))
 
 	RET()