Auto merge of #116505 - saethlin:infer-inline, r=cjgillot

Automatically enable cross-crate inlining for small functions

This is basically reviving https://github.com/rust-lang/rust/pull/70550

The `#[inline]` attribute can have a significant impact on code generation or runtime performance (because it enables inlining between CGUs where it would normally not happen) and also on compile-time performance (because it enables MIR inlining). But it has to be added manually, which is awkward.

This PR factors whether a DefId is cross-crate inlinable into a query, and replaces all uses of `CodegenFnAttrs::requests_inline` with this new query. The new query incorporates all the other logic that is used to determine whether a Def should be treated as cross-crate-inlinable, and as a last step inspects the function's optimized_mir to determine if it should be treated as cross-crate-inlinable.

The heuristic implemented here is deliberately conservative; we only infer inlinability for functions whose optimized_mir does not contain any calls or asserts. I plan to study adjusting the cost model later, but for now the compile time implications of this change are so significant that I think this very crude heuristic is well worth landing.

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