Implement PR8644: forwarding a memcpy value to a byval,

Implement PR8644: forwarding a memcpy value to a byval,
allowing the memcpy to be eliminated.

Unfortunately, the requirements on byval's without explicit
alignment are really weak and impossible to predict in the
mid-level optimizer, so this doesn't kick in much with current
frontends.  The fix is to change clang to set alignment on all
byval arguments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119916 91177308-0d34-0410-b5e6-96231b3b80d8