cmd/compile: on AMD64, prefer XOR/AND for (x & 1) == 0 check

cmd/compile: on AMD64, prefer XOR/AND for (x & 1) == 0 check

It's shorter to encode. Additionally, XOR and AND generally
have higher throughput than BT/SET*.

compilecmp:

runtime
runtime.(*sweepClass).split 58 -> 56  (-3.45%)
runtime.sweepClass.split 14 -> 11  (-21.43%)

runtime [cmd/compile]
runtime.(*sweepClass).split 58 -> 56  (-3.45%)
runtime.sweepClass.split 14 -> 11  (-21.43%)

strconv
strconv.ryuFtoaShortest changed

strconv [cmd/compile]
strconv.ryuFtoaShortest changed

math/big
math/big.(*Int).MulRange 255 -> 252  (-1.18%)

testing/quick
testing/quick.sizedValue changed

internal/fuzz
internal/fuzz.(*pcgRand).bool 69 -> 70  (+1.45%)

cmd/internal/obj/x86
cmd/internal/obj/x86.(*AsmBuf).asmevex changed

math/big [cmd/compile]
math/big.(*Int).MulRange 255 -> 252  (-1.18%)

cmd/internal/obj/x86 [cmd/compile]
cmd/internal/obj/x86.(*AsmBuf).asmevex changed

net/http
net/http.(*http2stream).isPushed 11 -> 10  (-9.09%)

cmd/vendor/github.com/google/pprof/internal/binutils
cmd/vendor/github.com/google/pprof/internal/binutils.(*file).computeBase changed

Change-Id: I9cb2987eb263c85ee4e93d6f8455c91a55273173
Reviewed-on: https://go-review.googlesource.com/c/go/+/640975
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>