cmd/compile: fix conditional select rule

ARM64 maintains booleans in the low byte of registers. Upper parts
of that register are junk.
This rule is using all 32 bits of a boolean-containing register, which
is wrong. Change the rule to only look at the low bit.

Fixes #57184

Change-Id: Ibbef86b2be859df3d06d993db00e1231c481c428
Reviewed-on: https://go-review.googlesource.com/c/go/+/456556
Auto-Submit: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

test/fixedbugs/issue57184.go

@@ -0,0 +1,40 @@
+// run
+
+// Copyright 2022 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 main
+
+import (
+	"log"
+	"reflect"
+	"sort"
+)
+
+func main() {
+	const length = 257
+	x := make([]int64, length)
+	for i := 0; i < length; i++ {
+		x[i] = int64(i) * 27644437 % int64(length)
+	}
+
+	isLessStatic := func(i, j int) bool {
+		return x[i] < x[j]
+	}
+
+	isLessReflect := reflect.MakeFunc(reflect.TypeOf(isLessStatic), func(args []reflect.Value) []reflect.Value {
+		i := args[0].Int()
+		j := args[1].Int()
+		b := x[i] < x[j]
+		return []reflect.Value{reflect.ValueOf(b)}
+	}).Interface().(func(i, j int) bool)
+
+	sort.SliceStable(x, isLessReflect)
+
+	for i := 0; i < length-1; i++ {
+		if x[i] >= x[i+1] {
+			log.Fatalf("not sorted! (length=%v, idx=%v)\n%v\n", length, i, x)
+		}
+	}
+}