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

These intrinsics are used to implement:
* svptrue_c8(), svptrue_c16(), etc.
* svcntp_c8(svcount_t pnn, uint64_t vl), svcntp_c16(...), etc.

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

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D150263