[SVE] Lower vector CTLZ, CTPOP and CTTZ operations.

CTLZ and CTPOP are lowered to CLZ and CNT instructions respectively.

CTTZ is not a native SVE operation but is instead lowered to:
  CTTZ(V) => CTLZ(BITREVERSE(V))

In the case of fixed-length support using SVE we also lower CTTZ
operating on NEON sized vectors because of its reliance on
BITREVERSE which is also lowered to SVE intructions at these lengths.

Differential Revision: https://reviews.llvm.org/D93607