[AArch64] Remove copy in SVE/SME predicate spill and fill (#81716)

7dc20abe introduced an extra COPY when spilling and filling a PNR
register, which can't be elided as the input (PNR predicate) and output
(PPR predicate) register classes differ. The patch adds a new register
class that covers both PPR and PNR so that STR_PXI and LDR_PXI can
take either of them, removing the need for the copy.