[AArch64][SME] Disable remat of VL-dependent ops when function changes streaming mode.

This is a way to prevent the register allocator from inserting instructions
which behave differently for different runtime vector-lengths, inside a
call-sequence which changes the streaming-SVE mode before/after the call.

I've considered using BUNDLEs in Machine IR, but found that using this is
not possible for a few reasons:
* Most passes don't look inside BUNDLEs, but some passes would need to
  look inside these call-sequence bundles, for example the PrologEpilog
  pass (to remove the CALLSEQSTART/END), a PostRA pass to remove COPY
  instructions, or the AArch64PseudoExpand pass.
* Within the streaming-mode-changing call sequence, one of the instructions
  is a CALLSEQEND. The corresponding CALLSEQBEGIN (AArch64::ADJCALLSTACKUP)
  is outside this sequence. This means we'd end up with a BUNDLE that has
  [SMSTART, COPY, BL, ADJCALLSTACKUP, COPY, SMSTOP]. The MachineVerifier
  doesn't accept this, and we also can't move the CALLSEQSTART into the
  call sequence.

Maybe in the future we could model this differently by modelling
the runtime vector-length as a value that's used by certain operations
(similar to e.g. NCZV flags) and clobbered by SMSTART/MMSTOP, such that the
register allocator can consider these as actual dependences and avoid
rematerialization. For now we just want to address the immediate problem.

Reviewed By: paulwalker-arm, aemerson

Differential Revision: https://reviews.llvm.org/D159193

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