Add type family performance test for #26426

sheafrequested to merge
sheaf/ghc:T26426 into master

This MR simply adds a performance test for #26426 (closed). The regression reported in that ticket has been (accidentally) fixed by 22d11fa8 and 0b7df6db, so we add a test to make sure we never re-introduce the regression.

TODO:

  • Refer to the performance test and the significant improvement in the respective Notes of the commits mentioned above.
Edited by sheaf

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