cmd/compile: optimize bitset tests

The assembly output for x & c == c, where c is power of 2:

	MOVQ	"".set+8(SP), AX
	ANDQ	$8, AX
	CMPQ	AX, $8
	SETEQ	"".~r2+24(SP)

With optimization using bitset:

	MOVQ	"".set+8(SP), AX
	BTL	$3, AX
	SETCS	"".~r2+24(SP)

output less than 1 instruction.

However, there is no speed improvement:

name         old time/op  new time/op  delta
AllBitSet-8  0.35ns ± 0%  0.35ns ± 0%   ~     (all equal)

Fixes #31904

Change-Id: I5dca4e410bf45716ed2145e3473979ec997e35d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/175957


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>