[release-branch.go1.11] cmd/compile: count nil check as use in dead auto elim

Nil check is special in that it has no use but we must keep it.
Count it as a use of the auto.

Fixes #27342.

Change-Id: I857c3d0db2ebdca1bc342b4993c0dac5c01e067f
Reviewed-on: https://go-review.googlesource.com/131955


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit 54f9c041)
Reviewed-on: https://go-review.googlesource.com/134615


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>