memcpy lowering code; this ensures that the size node has the desired
result type. This fixes a regression from r49572 with @llvm.memcpy.i64
on x86-32.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49761 91177308-0d34-0410-b5e6-96231b3b80d8