The LIT test cases were migrated with the script provided by
Nikita Popov.
No manual changes were made. Committed without review since
no functional changes, after consultation with uweigand.
The incoming back chain slot was implicitly allocated whenever a GPR was
saved in SystemZFrameLowering::getRegSpillOffset(), but in cases where no
GPRs were saved/restored this did not take effect.
Review: Ulrich Weigand
Differential Revision: https://reviews.llvm.org/D75367