[dev.regabi] cmd/compile: move okforconst into its own declaration

It needs to move into package ir, and we do not want all the rest.

Change-Id: Ibcfa1ebc0e63fe3659267bf2fa7069e8a93de4e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/272930


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