go/ssa: add position info to all Load/Store instructions

Was originally https://codereview.appspot.com/92880043/ by Richard Musiol.

Change-Id: If0b22cf962b94ef44edbac28a5e5af4acb950991
Reviewed-on: https://go-review.googlesource.com/2174
Reviewed-by: Robert Griesemer <gri@golang.org>