perf: use `hasExprMvar` in type class search

[JovanGerb]

I'm wondering if this change has a significant effect on Mathlib, because there might be lots of universe metavariables flying around there that we don't care about.

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Closes #0000 (RFC or bug issue number fixed by this PR, if any)

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❌ Mathlib branch lean-pr-testing-4276 built against this PR, but testing failed. (2024-05-25 04:41:26) View Log

[kim-em]

The spurious Mathlib failure from lake being noisy in tests should hopefully be resolved by tomorrow.