[AArch64]SME2 multi-vec FP/INT down convert 2/4 registers

[AArch64]SME2 multi-vec FP/INT  down convert 2/4 registers

This patch implements
Int:
  SQCVT: Multi-vector signed saturating extract narrow for 2 and 4 registers.
  UQCVT: Multi-vector unsigned saturating extract narrow for 2 and 4 registers.
  SQCVTU: Multi-vector signed saturating unsigned extract narrow for 2 and 4 registers
  SQCVTN:  Multi-vector signed saturating extract narrow and interleave.
  SQCVTUN: Multi-vector signed saturating unsigned extract narrow and interleave.
  UQCVTN: Multi-vector unsigned saturating extract narrow and interleave.

FP:
  FCVT(narrowing): Multi-vector floating-point convert from single-precision to
                  packed half-precision.
  FCVTN: Multi-vector floating-point convert from single-precision to
           interleaved half-precision.
  BFCVT: Multi-vector floating-point convert from single-precision to packed
         BFloat16 format.
  BFCVTN: : Multi-vector floating-point convert from single-precision to
            interleaved BFloat16 format.

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

Depends on: D135563

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D135588