cmd/compile/internal/gc: skip autotmp vars in gc again

cmd/compile/internal/gc: skip autotmp vars in gc again

Instead of skipping them based on string matching much later in the
compilation process, skip them up front using the proper API.

Passes toolstash-check.

Change-Id: Ibd4c0448a0701ba0de3235d4689ef300235fa1d9
Reviewed-on: https://go-review.googlesource.com/37930
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>