[EarlyCSE] Handle masked loads and stores

Extend the handling of memory intrinsics to also include non-
target-specific intrinsics, in particular masked loads and stores.

Invent "isHandledNonTargetIntrinsic" to distinguish between intrin-
sics that should be handled natively from intrinsics that can be
passed to TTI.

Add code that handles masked loads and stores and update the
testcase to reflect the results.

Differential Revision: https://reviews.llvm.org/D87340