MEMSET / MEMCPY lowering bugs: we can't issue a single WORD / DWORD version of

rep/stos and rep/mov if the count is not a constant. We could do
  rep/stosl; and $count, 3; rep/stosb
For now, I will lower them to memset / memcpy calls. We will revisit this after
a little bit experiment.

Also need to take care of the trailing bytes even if the count is a constant.
Since the max. number of trailing bytes are 3, we will simply issue loads /
stores.

llvm-svn: 26517