Fix a fixme: allow memdep to see past read-only calls when doing

Fix a fixme: allow memdep to see past read-only calls when doing
load dependence queries.  This allows GVN to eliminate a few more
instructions on 403.gcc:

 152598 gvn    - Number of instructions deleted
  49240 gvn    - Number of loads deleted
after:
 153986 gvn    - Number of instructions deleted
  50069 gvn    - Number of loads deleted

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60786 91177308-0d34-0410-b5e6-96231b3b80d8