[EarlyCSE][OpaquePtr] Check access type when performing DSE

This will bail out on target specific intrinsics. If those are deemed
important enough for EarlyCSE to handle, we can augment MemIntrinsicInfo
with an access type for TargetTransformInfo::getTgtMemIntrinsic() to
handle.

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D120077