mir-opt: support "FileCheck"-like annotations

[RalfJung]

Ever since our mir-opt tests were changed to auto-blessing, we no longer have a way to say "but this part of the output must definitely look like that". This makes tests a lot more fragile. We should re-add some infrastructure like that.

LLVM has FileCheck for this. We also had something like that for mir-opt tests that we could reinstate (I think it was adequate for this job, it just didn't work well for large chunks of code).

Cc @rust-lang/wg-mir-opt

[oli-obk]

The old system was removed in d0e0f40#diff-03a0567fa80ca04ed5a55f9ac5c711b4f84659be2d0ac4a984196d581c04f76b, so we should be able to revert that commit with some minor adjustments