[ARM] Fold away unnecessary CSET/CMPZ

Codegen from expanded vector operations can end up with unnecessary
CMPZ/CSINC, of the form:
  CSXYZ A, B, C1 (CMPZ (CSINC 0, 0, C2, D), 0)

These can be converted to remove the CMPZ and CSINC, depending on the
condition.
  if C1==NE -> CSXYZ A, B, C2, D
  if C1==EQ -> CSXYZ A, B, NOT(C2), D

Differential Revision: https://reviews.llvm.org/D114013