only rerun const eval in next-solver if the const actually references opaques

[sjwang05]

Split off from #161274: #161274 (comment)

Previously, we would unconditionally rerun eval if we were in ErasedNotCoherence mode, even if the const's eval result wouldn't depend on the TypingMode. This caused a slowdown with large types whose nested goals contained normalization goals involving types with anon assoc consts, since normalizing needs to evaluate those consts, which can't be done in ErasedNotCoherence mode, causing those goals, and all their parent goals, to be reevaluated.

For instance, if we had a large tuple with N elements in a goal, and each of the N elements had such a normalization goal, this would cause us to go quadratic, as is the case here: rust-lang/trait-system-refactor-initiative#272 (comment)

With this PR, we only retry const eval if the const's generics actually depend on any opaques which ErasedNotCoherence would influence.

next-solver is still 5x-ish slower than old solver on rust-lang/trait-system-refactor-initiative#272 (comment) due to some other ✨ Hidden Quadratics somewhere, but this PR improves perf on that specific reproducer by about 3x.

cc @lcnr, @jdonszelmann

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