Add support for alternative register names, useful for instructions whose operands...

Add support for alternative register names, useful for instructions whose operands are logically equivalent to existing registers, but happen to be printed specially.  For example, an instruciton that prints d0[0] instead of s0.
Patch by Jim Grosbach.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133940 91177308-0d34-0410-b5e6-96231b3b80d8