[dev.ssa] cmd/compile/internal/ssa: enforce load-store ordering in scheduler

[dev.ssa] cmd/compile/internal/ssa: enforce load-store ordering in scheduler

We must make sure that all loads that use a store are scheduled
before the next store.  Add additional dependency edges to the
value graph to enforce this constraint.

Change-Id: Iab83644f68bc4c30637085b82ca7467b9d5513a5
Reviewed-on: https://go-review.googlesource.com/13470
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>