sequence of loads and stores was being generated to perform the
copy on the x86 targets if the parameter was less than 4 byte
aligned, causing llc to use up vast amounts of memory and time.
Use a "rep movs" form instead. PR7170.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118260 91177308-0d34-0410-b5e6-96231b3b80d8
