[LegalizeDAG] Optimize CodeGen for `ISD::CTLZ_ZERO_UNDEF` (#83039) Previously we had the same instructions being generated for `ISD::CTLZ` and `ISD::CTLZ_ZERO_UNDEF` which did not take advantage of the fact that zero is an invalid input for `ISD::CTLZ_ZERO_UNDEF`. This commit separates codegen for the two cases to allow for the optimization for the latter case. The details of the optimization are outlined in #82075 Fixes #82075 Co-authored-by: Manish Kausik H <hmamishkausik@gmail.com>
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