cmd/gc: eliminate redundant &x.Field nil checks

This eliminates ~75% of the nil checks being emitted,
on all architectures. We can do better, but we need
a bit more general support from the compiler, and
I don't want to do that so close to Go 1.2.
What's here is simple but effective and safe.

A few small code generation cleanups were required
to make the analysis consistent on all systems about
which nil checks are omitted, at least in the test.

Fixes issue 6019.

R=ken2
CC=golang-dev
https://codereview.appspot.com/13334052