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Avoid allocations in `splitAtList` (#18535)

Sylvain Henry requested to merge hsyl20/ghc:hsyl20/perf/splitAtList into master

Avoid allocations in splitAtList (#18535)

As suspected by @simonpj in #18535, avoiding allocations in GHC.Utils.Misc.splitAtList when there are no leftover arguments is beneficial for performance:

On CI validate-x86_64-linux-deb9-hadrian:

    T12227 -7%
    T12545 -12.3%
    T5030  -10%
    T9872a -2%
    T9872b -2.1%
    T9872c -2.5%
Edited by Sylvain Henry

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