cmd/5g: fix register opt bug

cmd/5g: fix register opt bug

The width was not being set on the address, which meant
that the optimizer could not find variables that overlapped
with it and mark them as having had their address taken.
This let to the compiler believing variables had been set
but never used and then optimizing away the set.

Fixes #4129.

R=ken2
CC=golang-dev
https://golang.org/cl/6552059