[SystemZ] SystemZElimCompare pass improved.

Compare elimination extended to recognize load-and-test instructions used
for comparison and eliminate them the same way as with compare instructions.

Test case fp-cmp-05.ll updated to expect optimized results now also for z13.

The order of instruction shortening and compare elimination passes have been
changed so that opcodes do not have to be handled in both passes.

Reviewed by Ulrich Weigand.

llvm-svn: 249666