[InstCombine] Preserve llvm.mem.parallel_loop_access metadata when replacing

memcpy with ld/st.

When InstCombine replaces a memcpy with loads+stores it does not copy over the
llvm.mem.parallel_loop_access from the memcpy instruction. This patch fixes
that.

Differential Revision: https://reviews.llvm.org/D23499

llvm-svn: 280617