[mlir][Linalg] Add a useLinalgCopy option to Linalg bufferization.

Benchmarks show that memref::CopyOp is curently up to 200x slower than
tiled and vectorized versions of linalg::Copy.
Add a temporary flag to allow comprehensive bufferize to generate a
linalg::GenericOp that implements a copy until this performance bug is
resolved.

Differential Revision: https://reviews.llvm.org/D117696