5g: fix optimizer bug

5g: fix optimizer bug

same as in issue below, never fixed on ARM

changeset:   5498:3fa1372ca694
user:        Ken Thompson <ken@golang.org>
date:        Thu May 20 17:31:28 2010 -0700

description:
fix issue 798
cannot allocate an audomatic temp
while real registers are allocated.
there is a chance that the automatic
will be allocated to one of the
allocated registers. the fix is to
not registerize such variables.

R=rsc
CC=golang-dev
https://golang.org/cl/1202042

R=ken2
CC=golang-dev
https://golang.org/cl/4226042