[mlir][bufferize] Allow in-place bufferization for writes to init_tensors in loops

This commit relaxes the rules around ops that define a value but do not specify the tensor's contents. (The only such op at the moment is init_tensor.)

When such a tensor is written in a loop, it should not cause out-of-place bufferization.

Differential Revision: https://reviews.llvm.org/D124849