cmd/compile: use cheaper implementation of oneBit

This is the second attempt. The first attempt was CL 229127,
which got rolled back by CL 229177, because it caused
an infinite loop during compilation on some platforms.
I didn't notice that the trybots hadn't completed when I submitted; mea culpa.

The bug was that we were checking x&(x-1)==0, which is also true of 0,
which does not have exactly one bit set.
This caused an infinite rewrite rule loop.

Updates #38547

file    before    after     Δ       %
compile 19678112  19669808  -8304   -0.042%
total   113143160 113134856 -8304   -0.007%

Change-Id: I417a4f806e1ba61277e31bab2e57dd3f1ac7e835
Reviewed-on: https://go-review.googlesource.com/c/go/+/229197
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>

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

@@ -397,11 +397,11 @@ func ntz32(x int32) int { return bits.TrailingZeros32(uint32(x)) }
 func ntz16(x int16) int { return bits.TrailingZeros16(uint16(x)) }
 func ntz8(x int8) int   { return bits.TrailingZeros8(uint8(x)) }
 
-func oneBit(x int64) bool   { return bits.OnesCount64(uint64(x)) == 1 }
-func oneBit8(x int8) bool   { return bits.OnesCount8(uint8(x)) == 1 }
-func oneBit16(x int16) bool { return bits.OnesCount16(uint16(x)) == 1 }
-func oneBit32(x int32) bool { return bits.OnesCount32(uint32(x)) == 1 }
-func oneBit64(x int64) bool { return bits.OnesCount64(uint64(x)) == 1 }
+func oneBit(x int64) bool   { return x&(x-1) == 0 && x != 0 }
+func oneBit8(x int8) bool   { return x&(x-1) == 0 && x != 0 }
+func oneBit16(x int16) bool { return x&(x-1) == 0 && x != 0 }
+func oneBit32(x int32) bool { return x&(x-1) == 0 && x != 0 }
+func oneBit64(x int64) bool { return x&(x-1) == 0 && x != 0 }
 
 // nlo returns the number of leading ones.
 func nlo(x int64) int64 {