cmd/compile/internal/ssa: fix endless compile loop on AMD64

We currently rewrite
(TESTQ (MOVQconst [c] x)) into (TESTQconst [c] x)
and (TESTQconst [-1] x) into (TESTQ x x)
if x is a (MOVQconst [-1]) we will be stuck in the endless rewrite loop.
Don't perform the rewrite in such cases.

Fixes #25006

Change-Id: I77f561ba2605fc104f1e5d5c57f32e9d67a2c000
Reviewed-on: https://go-review.googlesource.com/108879
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue25006.go

@@ -0,0 +1,30 @@
+// compile
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func spin() {
+	var i int
+	var b bool
+
+	switch 1 {
+	case 0:
+		i = 1
+	}
+	switch 1 {
+	case i:
+	default:
+		i = 1
+		b = !b && (b && !b) && b
+	}
+	switch false {
+	case false:
+		i = 3 + -i
+		switch 0 {
+		case 1 - i:
+		}
+	}
+}