[CodeGen][AArch64][SVE] Fold [rdffr, ptest] => rdffrs; bugfix for optimizePTestInstr

[CodeGen][AArch64][SVE] Fold [rdffr, ptest] => rdffrs; bugfix for optimizePTestInstr

When a ptest is used to set flags from the output of rdffr, the ptest
can be eliminated, using a flags-setting rdffrs instead.

Additionally, check that nothing consumes flags between rdffr and ptest;
this case appears to have been missed previously.

* There is no unpredicated RDFFRS instruction.
* If substituting RDFFR_PP, require that the mask argument of the
  PTEST matches that of the RDFFR_PP.
* Move some precondition code up inside optimizePTestInstr, so that it
  covers the new code paths for RDFFR which return earlier.
  * Only consider RDFFR, PTEST in same basic block.
  * Check for other flag setting instructions between the two, abort if
    found.
  * Drop an old TODO comment about removing dead PTEST instructions.

RDFFR_P to follow in later patch.

Differential Revision: https://reviews.llvm.org/D101357