Recommit "[mips] Add names and tests for the hardware registers"

The original commit r221299 was reverted in r221307.  I removed the name
"hrw_ulr" ($29) from the original commit because two tests were failing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221681 91177308-0d34-0410-b5e6-96231b3b80d8