cmd/compile: use cheaper implementation of oneBit

Updates #38547

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

Change-Id: I5f8afe17401dbdb7c7b3d66d95fe40821c499a92
Reviewed-on: https://go-review.googlesource.com/c/go/+/229127
Run-TryBot: Josh Bleecher Snyder <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: josharian

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

@@ -395,11 +395,11 @@ 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 }
+func oneBit8(x int8) bool   { return x&(x-1) == 0 }
+func oneBit16(x int16) bool { return x&(x-1) == 0 }
+func oneBit32(x int32) bool { return x&(x-1) == 0 }
+func oneBit64(x int64) bool { return x&(x-1) == 0 }
 
 // nlo returns the number of leading ones.
 func nlo(x int64) int64 {