[AArch64][SVE2] Add the SVE2.1 while & pext predicate pair instructions

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

pext:    Set pair of predicates from predicate-as-counter
whilelt: While incrementing signed scalar less than scalar
whilele: While incrementing signed scalar less than or equal to scalar
whilegt: While incrementing signed scalar greater than scalar
whilege: While incrementing signed scalar greater than or equal to scalar
whilelo: While incrementing unsigned scalar lower than scalar
whilels: While incrementing unsigned scalar lower or same as scalar
whilehs: While decrementing unsigned scalar higher or same as scalar
whilehi: While decrementing unsigned scalar higher than scalar

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

Differential Revision: https://reviews.llvm.org/D136759