[dev.regabi] cmd/compile: remove refersToCommonName

After reorder3's simplification, the only remaining use of
refersToCommonName is in oaslit, where the LHS expression is always a
single name. We can replace the now overly-generalized
refersToCommonName with a simple ir.Any traversal with ir.Uses.

Passes toolstash -cmp.

Change-Id: Ice3020cdbbf6083d52e07866a687580f4eb134b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/280439


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>