[AArch64][SVE] Add structured load/store opcodes to getMemOpInfo

Currently, loading from or storing to a stack location with a structured load
or store crashes in isAArch64FrameOffsetLegal as the opcodes are not handled by
getMemOpInfo. This patch adds the opcodes for structured load/store instructions
with an immediate index to getMemOpInfo & getLoadStoreImmIdx, setting appropriate
values for the scale, width & min/max offsets.

Reviewed By: sdesmalen, david-arm

Differential Revision: https://reviews.llvm.org/D119338

(cherry picked from commit fc1b2122)