[dev.regabi] cmd/compile: add ir.BasicLit to represent literals

This CL changes so that all literals are represented with a new,
smaller ir.BasicLit type, so that ir.Name is only used to represent
declared constants.

Passes buildall w/ toolstash -cmp.

Change-Id: I4702b8e3fa945617bd05881d7a2be1205f229633
Reviewed-on: https://go-review.googlesource.com/c/go/+/279153


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>