cmd/compile: make isUint{32,64}PowerOfTwo implementations clearer

Since these functions cast the input to uint64, so the result always
non-negative. The condition should be changed to comparing with zero,
thus maaking it clearer to reader, and open room for simplifying in the
future by using the generic isUnsignedPowerOfTwo function.

Separated this change, so it's easier to do bisecting if there's any
problems happened.

Change-Id: Ibec28c2590f4c52caa36384b710d526459725e49
Reviewed-on: https://go-review.googlesource.com/c/go/+/692915
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Cuong Manh Le <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: cuonglm

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

@@ -507,13 +507,13 @@ func isUnsignedPowerOfTwo[T uint8 | uint16 | uint32 | uint64](n T) bool {
 // isUint64PowerOfTwo reports whether uint64(n) is a power of 2.
 func isUint64PowerOfTwo(in int64) bool {
 	n := uint64(in)
-	return n > 0 && n&(n-1) == 0
+	return n != 0 && n&(n-1) == 0
 }
 
 // isUint32PowerOfTwo reports whether uint32(n) is a power of 2.
 func isUint32PowerOfTwo(in int64) bool {
 	n := uint64(uint32(in))
-	return n > 0 && n&(n-1) == 0
+	return n != 0 && n&(n-1) == 0
 }
 
 // is32Bit reports whether n can be represented as a signed 32 bit integer.