[SystemZ] Add a mapping from "select register" to "load on condition" (2-addr).

The SELR(Mux) instructions can be converted to two-address form as LOCR(Mux)
instructions whenever one of the sources are the same reg as dest. By adding
this mapping in getTwoOperandOpcode(), we get:

- Two-address hints in getRegAllocationHints() for select register
  instructions.

- No need anymore for special handling in SystemZShortenInst.cpp -
  shortenSelect() removed.

The two-address hints are now added before the GRX32 hints, which should be
preferred.

Review: Ulrich Weigand
https://reviews.llvm.org/D68870