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
+}