Teach GVN to invalidate some memdep information when it does an RAUW

of a pointer.  This allows is to catch more equivalencies.  For example,
the type_lists_compatible_p function used to require two iterations of
the gvn pass (!) to delete its 18 redundant loads because the first pass
would CSE all the addressing computation cruft, which would unblock the
second memdep/gvn passes from recognizing them.  This change allows
memdep/gvn to catch all 18 when run just once on the function (as is 
typical :) instead of just 3.

On all of 403.gcc, this bumps up the # reundandancies found from:

     63 gvn    - Number of instructions PRE'd
 153991 gvn    - Number of instructions deleted
  50069 gvn    - Number of loads deleted
to:
     63 gvn    - Number of instructions PRE'd
 154137 gvn    - Number of instructions deleted
  50185 gvn    - Number of loads deleted

+120 loads deleted isn't bad.

llvm-svn: 60799