Introduced in CL 9263 (prepare to unexport gc.Mp*) and CL 9267
(prepare Node.Val to be unexported), their only callers were in
the old backend and all got deleted in CL 29168 (cmd/compile:
delete lots of the legacy backend).

Update #16357

Change-Id: I0a5d76b98b418e8ec0984c033c3bc0ac3fc5f38a
Reviewed-on: https://go-review.googlesource.com/29997Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>