[dev.regabi] cmd/compile: refactor temp construction in walk

This CL adds a few new helper functions for constructing and
initializing temporary variables during walk.

Passes toolstash -cmp.

Change-Id: I54965d992cd8dfef7cb7dc92a17c88372e52a0d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/284224


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>