CodeGen: Copy tail padding when we're not dealing with a trivial copy assign or move...

CodeGen: Copy tail padding when we're not dealing with a trivial copy assign or move assign operator.

This fixes a regression from r162254, the optimizer has problems reasoning
about the smaller memcpy as it's often not safe to widen a store but making it
smaller is.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164917 91177308-0d34-0410-b5e6-96231b3b80d8