Rollup merge of #136278 - lqd:polonius-debugger-episode-3, r=matthewjasper

add constraint graph to polonius MIR dump

Another easy one while I work on diagnostics. This PR adds a mermaid visualization of the polonius constraint graph to the polonius MIR dump.

Adding kills is left to a future PR (until they're encoded in edges directly or I set up recording debugging info in and out of the analysis), because right now they're only computed during traversal.

[Here's](https://gistpreview.github.io/?096b0131e8258f9a3125c55c7ac369bc) how that looks.

 r? `@matthewjasper` but as always feel free to reroll.

Author: jhpratt