[AArch64][SVE] Add support for spilling/filling ZPR2/3/4

Summary:
This patch enables the register allocator to spill/fill lists of 2, 3
and 4 SVE vectors registers to/from the stack. This is implemented with
pseudo instructions that get expanded to individual LDR_ZXI/STR_ZXI
instructions in AArch64ExpandPseudoInsts.

Patch by Sander de Smalen.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75988