cmd/compile: fix load combining from a non-zero pointer offset

cmd/compile: fix load combining from a non-zero pointer offset

When the pointer offset is non-zero in the small loads, we need to add the offset
when converting to the larger load.

Fixes #14694

Change-Id: I5ba8bcb3b9ce26c7fae0c4951500b9ef0fed54cd
Reviewed-on: https://go-review.googlesource.com/20333
Reviewed-by: Keith Randall <khr@golang.org>