inst,ir: cancelling inputs (#92)

Adds support for a `CancellingInputs` instruction flag, to indicate cases like `XORQ R10, R10` where the instruction actually does not depend on the value of `R10` at all.

Closes #89

tests/fixedbugs/issue89/asm.go

@@ -0,0 +1,22 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Issue89", NOSPLIT, "func() uint64")
+	x := GP64()
+	MOVQ(U32(42), x)
+	for i := 0; i < 100; i++ {
+		zero := GP64()
+		XORQ(zero, zero)
+		ADDQ(zero, x)
+	}
+	Store(x, ReturnIndex(0))
+	RET()
+	Generate()
+}