[dev.regabi] cmd/compile: simplify stack temp initialization

This CL simplifies the previous one a little bit further, by combining
reordering stack-temporary initialization and getting rid of an
unneeded temporary variable. (Does not pass toolstash -cmp.)

Change-Id: I17799dfe368484f33a8ddd0ab4f68647d6262147
Reviewed-on: https://go-review.googlesource.com/c/go/+/284225


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>