[SystemZ] Improve handling of backchain offset.

- New function SDValue getBackchainAddress() used by
  lowerDYNAMIC_STACKALLOC() and lowerSTACKRESTORE() to properly handle the
  backchain offset also with packed-stack.

- Make a common function getBackchainOffset() for the computation of the
  backchain offset and use in some places (NFC).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D93171