perf: assign synthesized instances directly when the goal type is ground (#14330)

This PR makes `tryResolve` assign the goal metavariable directly after
successfully unifying the goal type with the candidate instance type,
instead of re-checking the types with `isDefEq`. The recheck is
redundant for metavariable-free goals and can be expensive.

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>