cmd/compile: fix wrong comment message in AMD64Ops.go
cmd/compile: fix wrong comment message in AMD64Ops.go

According to AMD64.rules, BTS&BTR&BTC use arg1 as the bit index,
while BT uses arg0. This CL fixes the wrong comment message in
AMD64Ops.go, which indicates all bit indexes are in arg0.

Change-Id: Idb78f4d39f7ef5ea78065ad8bc651324597e2a8a
Reviewed-on: https://go-review.googlesource.com/135419
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

josharian/go,golang/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,josharian/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,golang/go,akutz/go,nawawi/go,josharian/go,nawawi/go,akutz/go,akutz/go,nawawi/go,akutz/go,golang/go,CAFxX/go,josharian/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,josharian/go,golang/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go