[VPlan] Fix crash when unrolling in-loop reduction chains (#129840)

If an in-loop reduction is chained e.g.

    WIDEN-REDUCTION-PHI ir<%rdx> = phi ir<0>, ir<%add2>
    REDUCE ir<%add1> = ir<%rdx> + reduce.add (ir<%x>)
    REDUCE ir<%add2> = ir<%add1> + reduce.add (ir<%y>)

When we try to unroll the second add reduction, we crash because we
currently expect the chain to be a VPReductionPHIRecipe, when in fact
it's the previous reduction. This relaxes the cast to a dyn_cast, so we
end up unrolling to:

    WIDEN-REDUCTION-PHI ir<%rdx> = phi ir<0>, ir<%add2>
    WIDEN-REDUCTION-PHI ir<%rdx>.1 = phi ir<0>, ir<%add2>.1, ir<1>
    WIDEN-REDUCTION-PHI ir<%rdx>.2 = phi ir<0>, ir<%add2>.2, ir<2>
    WIDEN-REDUCTION-PHI ir<%rdx>.3 = phi ir<0>, ir<%add2>.3, ir<3>
    REDUCE ir<%add1> = ir<%rdx> + reduce.add (ir<%x>)
    REDUCE ir<%add1>.1 = ir<%rdx>.1 + reduce.add (ir<%x>.1)
    REDUCE ir<%add1>.2 = ir<%rdx>.2 + reduce.add (ir<%x>.2)
    REDUCE ir<%add1>.3 = ir<%rdx>.3 + reduce.add (ir<%x>.3)
    REDUCE ir<%add2> = ir<%add1> + reduce.add (ir<%y>)
    REDUCE ir<%add2>.1 = ir<%add1>.1 + reduce.add (ir<%y>.1)
    REDUCE ir<%add2>.2 = ir<%add1>.2 + reduce.add (ir<%y>.2)
    REDUCE ir<%add2>.3 = ir<%add1>.3 + reduce.add (ir<%y>.3)

This fixes a crash when building 525.x264_r from SPEC CPU 2017 on
AArch64 with -mllvm -prefer-inloop-reductions

0 files changed, 0 insertions, 0 deletions