cmd/compile: make memcombine pass a bit more robust to reassociation of exprs

cmd/compile: make memcombine pass a bit more robust to reassociation of exprs

Be more liberal about expanding the OR tree. Handle any tree shape
instead of a fully left or right associative tree.

Also remove tail feature, it isn't ever needed.

Change-Id: If16bebef94b952a604d6069e9be3d9129994cb6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/494056
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ryan Berger <ryanbberger@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>