[mips][msa] CHECK-DAG-ize MSA elm_copy.ll test.

This update is a preparation for the addition of Mips64 MSA tests.

No functional changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200395 91177308-0d34-0410-b5e6-96231b3b80d8