cmd/compile: add code to expand calls just before late opt

Still needs to generate the calls that will need lowering.

Change-Id: Ifd4e510193441a5e27c462c1f1d704f07bf6dec3
Reviewed-on: https://go-review.googlesource.com/c/go/+/242359


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>