Auto merge of #113853 - cjgillot:split-validator, r=compiler-errors

Reuse the MIR validator for MIR inlining

Instead of having the inliner home-cook its own validation, we just check that the substituted MIR body passes the regular validation.

The MIR validation is first split in two: control flow validation (MIR syntax and CFG invariants) and type validation (subtyping relationship in assignments and projections). Only the latter can be affected by instantiating type parameters.

Author: bors