mir_build: Add an extra intermediate step in MIR building for patterns This is an attempt to partly decouple the data structures created by `match_pair.rs` from the data structures that are ultimately consumed by the main part of match lowering. In some ways this is a reversal from https://github.com/rust-lang/rust/pull/137875. That PR succeeded in removing the `TestCase::Irrefutable` variant, by taking a pre-existing “simplification” step and fusing it directly into `MatchPairTree::for_pattern`. Unfortunately, in doing so it also reinforced a very high degree of coupling between the transformations performed in `match_pair`, and the data structures used by later steps. My hope is that these changes will make it easier for follow-up work to further separate decision-making from MIR building when lowering patterns. r? Nadrieril