cmd/compile/internal/ssa: use math/bits in countRegs and pickReg

Makes code simpler and faster (at least on x86).

	name               old time/op  new time/op  delta
	CountRegs-8        7.40ns ± 1%  0.59ns ± 0%  -92.02%  (p=0.000 n=9+9)
	PickReg/(1<<0)-8   2.07ns ± 0%  0.37ns ± 0%  -82.13%  (p=0.000 n=9+10)
	PickReg/(1<<16)-8  11.8ns ± 0%   0.4ns ± 0%  -96.86%  (p=0.002 n=8+10)

Change-Id: Ic780b615b75c25b6e7632a0de93b16a8e9ed0f8f
Reviewed-on: https://go-review.googlesource.com/120318
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -119,6 +119,7 @@ import (
 	"cmd/internal/src"
 	"cmd/internal/sys"
 	"fmt"
+	"math/bits"
 	"unsafe"
 )
 
@@ -183,26 +184,16 @@ func (s *regAllocState) RegMaskString(m regMask) string {
 
 // countRegs returns the number of set bits in the register mask.
 func countRegs(r regMask) int {
-	n := 0
-	for r != 0 {
-		n += int(r & 1)
-		r >>= 1
-	}
-	return n
+	return bits.OnesCount64(uint64(r))
 }
 
 // pickReg picks an arbitrary register from the register mask.
 func pickReg(r regMask) register {
-	// pick the lowest one
 	if r == 0 {
 		panic("can't pick a register from an empty set")
 	}
-	for i := register(0); ; i++ {
-		if r&1 != 0 {
-			return i
-		}
-		r >>= 1
-	}
+	// pick the lowest one
+	return register(bits.TrailingZeros64(uint64(r)))
 }
 
 type use struct {