[SystemZ] Merge the SystemZExpandPseudo pass into SystemZPostRewrite.

SystemZExpandPseudo:s only job was to expand LOCRMux instructions into jump
sequences. This needs to be done if expandLOCRPseudo() or expandSELRPseudo()
fails to find a legal opcode (all registers "high" or "low"). This task has
now been moved to SystemZPostRewrite while removing the SystemZExpandPseudo
pass.

It is in fact preferred to expand these pseudos directly after register
allocation in SystemZPostRewrite since the hinted register combinations are
then not subject to later optimizations.

Review: Ulrich Weigand
https://reviews.llvm.org/D67432

llvm-svn: 371959