[SystemZ] Add `is(LoadFrom|StoreTo)StackSlotPostFE` to SystemZBackend (#132928)

As part of an effort to enable instr-ref-based debug value tracking,
this PR implements `SystemZInstrInfo::isLoadFromStackSlotPostFE`, as
well as `SystemZInstrInfo::isStoreToStackSlotPostFE`. The implementation
relies upon the presence of MachineMemoryOperands on the relevant
`MachineInstr`s in order to access the `FrameIndex` post frame index
elimination.

Since these new functions are only meant to be called after frame-index
elimination, they assert against the present of a frame index on the
base register operand of the instruction.

Outside of the utility of these functions to enable instr-ref-based
debug value tracking, they also changes the behavior of the AsmPrinter,
since it will now be able to properly detect non-folded spills and
reloads, so this changes a number of tests that were checking
specifically for folded reloads.

Note that there are some tests that still check for `vst` and `vl` as
folded spills/reloads even though they should be straight reloads. This
will be addressed in a future PR.

Co-authored-by: Dominik Steenken <dominik.steenken@gmail.com>

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