cmd/compile: collapse OffPtr sequences

cmd/compile: collapse OffPtr sequences

This triggers an astonishing 160k times
during make.bash. The second biggest
generic rewrite triggers 100k times.

However, this is really just moving
rewrites that were happening at the
architecture level to the generic level.

Change-Id: Ife06fe5234f31433328460cb2e0741c071deda41
Reviewed-on: https://go-review.googlesource.com/20235
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>