[mips] Add hardware register name "hwr_ulr" ($29)

The canonical name when printing assembly is still $29. The reason is that
GAS does not accept "$hwr_ulr" at the moment.

This addresses the comments from r221307, which reverted the original
commit r221299.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221685 91177308-0d34-0410-b5e6-96231b3b80d8