cmd/compile: fold boolean NOT into branches

Gets rid of an EOR $1 instruction.

Change-Id: Ib032b0cee9ac484329c978af9b1305446f8d5dac
Reviewed-on: https://go-review.googlesource.com/c/go/+/721501
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>

test/codegen/bool.go

@@ -313,3 +313,18 @@ func constantWrite(b bool, p *bool) {
 		*p = b
 	}
 }
+
+func boolCompare1(p, q *bool) int {
+	// arm64:-"EOR [$]1"
+	if *p == *q {
+		return 5
+	}
+	return 7
+}
+func boolCompare2(p, q *bool) int {
+	// arm64:-"EOR [$]1"
+	if *p != *q {
+		return 5
+	}
+	return 7
+}