The memcpy optimizer was happily doing call slot forwarding when the new memory

The memcpy optimizer was happily doing call slot forwarding when the new memory
was less aligned than the old.  In the testcase this results in an overaligned
memset: the memset alignment was correct for the original memory but is too much
for the new memory.  Fix this by either increasing the alignment of the new
memory or bailing out if that isn't possible.  Should fix the gcc-4.7 self-host
buildbot failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165220 91177308-0d34-0410-b5e6-96231b3b80d8