cmd/compile/internal/gc: cleanup mkinlcall

cmd/compile/internal/gc: cleanup mkinlcall

I had too many failed attempts trying to remove iterFields that I
decided to overhaul this function. Much simpler and easier to
understand now (at least IMO).

Passes toolstash-check -all.

Change-Id: I41d00642a969698df3f4689e41a386346b966638
Reviewed-on: https://go-review.googlesource.com/39856
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>