[mlir] Fix subtensor_insert bufferization.

It was incorrect in the presence of a tensor argument with multiple
uses.

The bufferization of subtensor_insert was writing into a converted
memref operand, but there is no guarantee that the converted memref for
that operand is safe to write into. In this case, the same converted
memref is written to in-place by the subtensor_insert bufferization,
violating the tensor-level semantics.

I left some comments in a TODO about ways forward on this. I will be
working actively on this problem in the coming days.

Differential Revision: https://reviews.llvm.org/D91371