cmd/compile: reuse auto symbols during ssa construction

Reuse auto symbols so cse can eliminate OpAddrs that refer to
them.

Change-Id: I69e6a3f77a3a33946459cf8c6eccf223f6125048
Reviewed-on: https://go-review.googlesource.com/20569


Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>