cmd/compile/internal/gc: minor Cgen_checknil cleanup

cmd/compile/internal/gc: minor Cgen_checknil cleanup

Most architectures can only generate nil checks when the
the address to check is in a register. Currently only
amd64 and 386 can generate checks for addresses that
reside in memory. This is unlikely to change so the architecture
check has been inverted.

Change-Id: I73697488a183406c79a9039c62823712b510bb6a
Reviewed-on: https://go-review.googlesource.com/21861
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>