[AArch64][SVE2] Add the SVE2.1 pext and ptrue predicate-as-counter instructions

[AArch64][SVE2] Add the SVE2.1 pext and ptrue predicate-as-counter instructions

This patch adds the assembly/disassembly for the following instructions:

pext (predicate) : Set predicate from predicate-as-counter
ptrue (predicate-as-counter) : Initialise predicate-as-counter to all active

This patch also introduces the predicate-as-counter registers pn8, etc.

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2022-09

Differential Revision: https://reviews.llvm.org/D136678