Rollup merge of #137875 - Zalathar:irrefutable, r=Nadrieril

mir_build: Integrate "simplification" steps into match-pair-tree creation

The “simplification” step helps to prepare THIR patterns for lowering into MIR, and originally dates back to the earliest days of MIR in the compiler.

Over time, various intermediate data structures have been introduced (e.g. `MatchPair`, later renamed to `MatchPairTree`) that reduce the need for a separate simplification step, because some of the necessary simplifications can be built into the construction of those intermediate structures instead. This PR continues that process to its logical conclusion and removes the simplification step entirely, by integrating its remaining responsibilities into match-pair-tree creation: flattening “irrefutable” nodes, collecting bindings/ascriptions in flat lists, and sorting or-patterns after other subpatterns.

This has a few immediate benefits:
- We can remove `TestCase::Irrefutable`, which was not allowed to exist after simplification, and was much larger than other test-case variants.
- We can make `MatchPairTree::place` non-optional, because only irrefutable nodes could fail to have a place.

In the future, this should also help with some ideas I have for simplifying how `AscribeUserType` and `ExpandedConstant` nodes are handled, by representing them as side-data keyed by THIR pattern ID, so that they are no longer their own kinds of THIR pattern node.

Author: jieyouxu