[AArch64] Add AArch64TTIImpl::getMaskedMemoryOpCost function

When vectorising for AArch64 targets if you specify the SVE attribute
we automatically then treat masked loads and stores as legal. Also,
since we have no cost model for masked memory ops we believe it's
cheap to use the masked load/store intrinsics even for fixed width
vectors. This can lead to poor code quality as the intrinsics will
currently be scalarised in the backend. This patch adds a basic
cost model that marks fixed-width masked memory ops as significantly
more expensive than for scalable vectors.

Tests for the cost model are added here:

  Transforms/LoopVectorize/AArch64/masked-op-cost.ll

Differential Revision: https://reviews.llvm.org/D100745