cmd/compile: expand calls cleanup

cmd/compile: expand calls cleanup

Convert expand calls into a smaller number of focused
recursive rewrites, and rely on an enhanced version of
"decompose" to clean up afterwards.

Debugging information seems to emerge intact.

Change-Id: Ic46da4207e3a4da5c8e2c47b637b0e35abbe56bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/507295
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>