[mlir][linalg][bufferize] LinalgOp: Move existing region to new op

This has two advantages.

1. It is more efficient. No need to clone the entire region.
2. Recreating ops (via cloning) invalidates analysis results. Previously, an OpResult could have bufferized out-of-place, even though the analysis requested an in-place bufferization. That is because BufferizationState keeps track of OpResults for storing bufferization analysis results (and cloned ops have new OpResults).

Differential Revision: https://reviews.llvm.org/D116453