[mlir][Linalg] Bufferize the region of LinalgOps as well.

The region of `linalg.generic` might contain `tensor` operations. For
example, current lowering of `gather` uses a `tensor.extract` in the
body of the `LinalgOp`. Bufferize the ops within a `LinalgOp` region
as well to catch such cases.

Differential Revision: https://reviews.llvm.org/D115322