cmd/compile: use OffPtr when possible

cmd/compile: use OffPtr when possible

OffPtr allocates less and is easier to optimize.

With this change, the OffPtr collapsing opt
rule matches increase from 160k to 263k,
and the Load-after-Store opt rule matches
increase from 217 to 853.

Change-Id: I763426a3196900f22a367f7f6d8e8047b279653d
Reviewed-on: https://go-review.googlesource.com/20273
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>
Reviewed-by: Keith Randall <khr@golang.org>