Instead of doing an explicit index in aarch64-fusion-pairs.def, we
    should have an enum which does the index instead.  This allows
    you to add/remove them without worrying about the order being
    correct and having holes or worry about merge conflicts.

    OK? Bootstrapped and tested on aarch64-linux-gnu with no regressions.

    ChangeLog:
    * aarch64-fusion-pairs.def: Remove all index to AARCH64_FUSION_PAIR.
    * config/aarch64/aarch64-protos.h (aarch64_fusion_pairs_index): New enum.
    (aarch64_fusion_pairs): Base the shifted value on the index instead
    of the argument to AARCH64_FUSION_PAIR.
    * config/aarch64/aarch64.c: Remove the last argument to AARCH64_FUSION_PAIR.