[dev.regabi] cmd/compile: rename a few 'base' identifiers

We want to introduce a package cmd/compile/internal/base,
and these will shadow it at points where it is needed.

Change-Id: Ic936733fba1ccba8c2ca1fdedbd4d2989df4bbf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/272249


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>