Auto merge of #128015 - Nadrieril:two-step-or-expansion, r=compiler-errors

match exhaustiveness: Expand or-patterns as a separate step

To compute exhaustiveness, we must expand or-patterns. Previously, we expanded them at the same time that we pushed patterns into the matrix. This made it harder to track pattern reachability, because the or-pattern itself would never show up in the matrix so we had to recover missing information.

This PR changes that: we no longer expand or-patterns as we push them into the matrix. Instead, if we find an or-pattern in the matrix we expand them in a step very much like the specialization we already do. This simplifies a bunch of things, and should greatly simplify the implementation of rust-lang/rust#127870.

r? `@compiler-errors`

Author: bors