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

These intrinsics are used to implement the while intrinsics that result
in a predicate-as-counter value, e.g.

  __attribute__((arm_streaming))
  svcount_t svwhilelt_c8(int64_t rn, int64_t rm, uint64_t vl)

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

Reviewed By: hassnaa-arm

Differential Revision: https://reviews.llvm.org/D150265