Turn SSAing of variables off when compiling with optimizations off.
This helps keep variable names around that would otherwise be
optimized away.

Fixes #14744

Change-Id: I31db8cf269c068c7c5851808f13e5955a09810ca
Reviewed-on: https://go-review.googlesource.com/22681
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>