[SystemZ] Fixes in the backend I/R.

expandPostRAPseudo():
STX -> 2 * STD: The first STD should not have the kill flag set for the address.

SystemZElimCompare:
BRC -> BRCT conversion: Don't forget to remove the CC<use,kill> operand.

Needed to make SystemZ/asm-17.ll pass with -verify-machineinstrs, which
now runs with this flag.

Reviewed by Ulrich Weigand.

llvm-svn: 249945