[PatternMatching] Add convenience insert method to OwningRewritePatternList. NFC.

This allows adding a C function pointer as a matchAndRewrite style pattern, which
is a very common case.  This adopts it in ExpandTanh to show how it reduces a level
of nesting.

We could allow C++ lambdas here, but that doesn't work as well with type inference
in the common case.  Instead of:

  patterns.insert(convertTanhOp);

you need to specify:

  patterns.insert<math::TanhOp>(convertTanhOp);

which is boilerplate'y.  Capturing state like this is very uncommon, so we choose
to require clients to define their own structs and use the non-convenience method
when they need to do so.

Differential Revision: https://reviews.llvm.org/D99039