[AArch64][SME2][SVE2p1] Add PNR_3b regclass (#67785)

This patch adds the PNR_3b regclass for predicate-as-counter registers
0-7 and allows the Upl ASM constraint to use this register class.

GitOrigin-RevId: 6f5b372d593f1d08f6569a32f529b6bd5106237c
9 files changed