cmd/compile: correct the argument type in SETXXstore -> MOVBstore rules on AMD64

MOVBstore's value argument is a value, not a flag. We are storing
a byte so just use UInt8.

Fixes #31915.

Change-Id: Id799e5f44efc3a9c3d8480f6f25ad032c2a631bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/176719
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue31915.go

+// compile -d=ssa/check/on
+
+// Copyright 2019 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 f()
+
+func g() {
+	var a []int
+	var b bool
+	for {
+		b = (b && b) != (b && b)
+		for b && b == b || true {
+			f()
+			_ = a[0]
+		}
+		_ = &b
+		a = []int{}
+	}
+}