Add an addition operator to TableGen

This adds an !add(a, b) operator to tablegen; this will be used
to cleanup the PPC register definitions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173445 91177308-0d34-0410-b5e6-96231b3b80d8
More cleanup of PPC register definitions.

Uses the new !add TableGen operator to do more cleanup of the
PPC register definitions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173446 91177308-0d34-0410-b5e6-96231b3b80d8