[AArch64][SME2/SVE2p1] Add predicate-as-counter intrinsics for sel

These intrinsics are used to implement the sel intrinsics that selects
a tuple of 2 or 4 values based on a predicate-as-counter operand, e.g.

  __attribute__((arm_streaming))
  svuint8x2_t svsel[_u8_x2](svcount_t png, svuint8x2_t zn, svuint8x2_t zm);

  __attribute__((arm_streaming))
  svuint8x4_t svsel[_u8_x4](svcount_t png, svuint8x4_t zn, svuint8x4_t zm);

As described in https://github.com/ARM-software/acle/pull/217

Reviewed By: CarolineConcatto

Differential Revision: https://reviews.llvm.org/D150951