[MLIR][LLVM][Mem2Reg] Relax type equality requirement for load and store

This commit relaxes Mem2Reg's type equality requirement for the LLVM
dialect's load and store operations. For now, we only allow loads to be
promoted if the reaching definition can be casted into a value of the
target type.

For stores, the same conversion casting check is applied and we ensure
that their result is properly casted to the type of the memory slot.
This is necessary to satisfy assumptions of the general mem2reg pass, as
it creates block arguments with the types of the memory slot.