cmd/compile: fix incorrect comparison folding

We lost a sign extension that was necessary. The nonnegative comparison
didn't have the correct extension on it. If the larger constant is
positive, but its shorter sign extension is negative, the rule breaks.

Fixes #41872

Change-Id: I6592ef103f840fbb786bf8cb94fd8804c760c976
Reviewed-on: https://go-review.googlesource.com/c/go/+/260701
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alberto Donizetti <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1274,8 +1274,8 @@
 (CMPQconst (ANDQconst _ [m]) [n]) && 0 <= m && m < n => (FlagLT_ULT)
 (CMPQconst (ANDLconst _ [m]) [n]) && 0 <= m && m < n => (FlagLT_ULT)
 (CMPLconst (ANDLconst _ [m]) [n]) && 0 <= m && m < n => (FlagLT_ULT)
-(CMPWconst (ANDLconst _ [m]) [n]) && 0 <= m && int16(m) < n => (FlagLT_ULT)
-(CMPBconst (ANDLconst _ [m]) [n]) && 0 <= m && int8(m)  < n => (FlagLT_ULT)
+(CMPWconst (ANDLconst _ [m]) [n]) && 0 <= int16(m) && int16(m) < n => (FlagLT_ULT)
+(CMPBconst (ANDLconst _ [m]) [n]) && 0 <= int8(m)  && int8(m)  < n => (FlagLT_ULT)
 
 // TESTQ c c sets flags like CMPQ c 0.
 (TESTQconst [c] (MOVQconst [d])) && int64(c) == d && c == 0 => (FlagEQ)

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -0,0 +1,26 @@
+// run
+
+// Copyright 2020 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
+
+//go:noinline
+func f8(x int32) bool {
+	return byte(x&0xc0) == 64
+}
+
+//go:noinline
+func f16(x int32) bool {
+	return uint16(x&0x8040) == 64
+}
+
+func main() {
+	if !f8(64) {
+		panic("wanted true, got false")
+	}
+	if !f16(64) {
+		panic("wanted true, got false")
+	}
+}