[mlir][linalg][transform] Rename ApplyPatternsOp.{region => patterns}.

This gives the region a more meaningful name. The topic came up in a
discussion on https://reviews.llvm.org/D155435, where the name `region`
would have led to a situation where a convenience accessor called
`region` (after the ODS name) would have returned a Block.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D155810