Add one target hook predict_doloop_p, it return true if we can predict it
    is possible to use a low-overhead loop, it can help ivopts to make some
    better decisions.

    PR middle-end/80791
    * target.def (predict_doloop_p): New hook.
    * targhooks.h (default_predict_doloop_p): New declaration.
    * targhooks.c (default_predict_doloop_p): New function.
    * doc/tm.texi.in (TARGET_PREDICT_DOLOOP_P): New hook.
    * doc/tm.texi: Regenerate.
    * config/rs6000/rs6000.c (rs6000_predict_doloop_p): New function.
    (TARGET_PREDICT_DOLOOP_P): New macro.
    * tree-ssa-loop-ivopts.c (generic_predict_doloop_p): New function.

From-SVN: r272405