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>