[EarlyCSE] Fix crash when optimizing masked loads/stores

With opaque pointers, it is possible for EarlyCSE to encounter masked
load/store intrinsics which access the same pointer value but with
different incompatible types. These cannot form valid replacements
(without explicit casting, which we don't yet do even for regular
load/store instructions) so should be prevented.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D141613