Auto merge of #142625 - cjgillot:inline-nocycle, r=oli-obk

Only compute recursive callees once.

Inlining MIR in a cyclic call graph may create query cycles, which are ICEs. The current implementation `mir_callgraph_reachable(inlining_candidate, being_optimized)` checks if calling `inlining_candidate` may cycle back to `being_optimized` that we are currently inlining into.

This PR replaces this device with query `mir_callgraph_cyclic(being_optimized)` which searches the call graph for all cycles going back to `being_optimized`, and returns the set of functions involved in those cycles.

This is a tradeoff:
- in the current implementation, we perform more walks, but shallower;
- in this new implementation, we perform fewer walks, but exhaust the graph.

I'd have liked to compute this using some kind of SCC, but generic parameters make resolution path-dependent, so usual graph algorithms do not apply.

Author: bors