[AArch64] Create proper memoperand for multi-vector stores

Re-apply r345315 with testcase fixes.

Include all of the store's source vector operands when creating the
MachineMemOperand. Previously, we were missing the first operand,
making the store size seem smaller than it really is.

Differential Revision: https://reviews.llvm.org/D52816

llvm-svn: 345631