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/issue89_test.go

@@ -0,0 +1,13 @@
+package issue89
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -out issue89.s -stubs stub.go
+
+func TestIssue89(t *testing.T) {
+	if Issue89() != 42 {
+		t.FailNow()
+	}
+}