[dev.regabi] cmd/compile: add ir.CallPartExpr

[dev.regabi] cmd/compile: add ir.CallPartExpr

Now there are no longer any generic nodes with a non-nil
associated Func, so node.fn can be deleted. Also all manipulation
of func fields is done with concrete types, so Node.SetFunc can be
deleted, along with generic implementations.

Passes buildall w/ toolstash -cmp.

Change-Id: I4fee99870951ec9dc224f146d87b22e2bfe16889
Reviewed-on: https://go-review.googlesource.com/c/go/+/274099
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>