- Simplify PatternMatch to *require* static benefits at pattern construction

  time.  The "Fast and Flexible Instruction Selection With Constraints" paper
  from CC2018 makes a credible argument that dynamic costs aren't actually
  necessary/important, and we are not using them.

- Check in my "MLIR Generic DAG Rewriter Infrastructure" design doc into the
  source tree.

PiperOrigin-RevId: 221017546