[dev.regabi] cmd/compile: only access Func method on concrete types

[dev.regabi] cmd/compile: only access Func method on concrete types

Sets up for removing Func from Node interface.
That means that once the Name reorg is done,
which will let us remove Name, Sym, and Val,
Node will be basically a minimal interface.

Passes buildall w/ toolstash -cmp.

Change-Id: I6e87897572debd7f8e29b4f5167763dc2792b408
Reviewed-on: https://go-review.googlesource.com/c/go/+/279484
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>