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.

llvm-svn: 119916