[DAG] canCreateUndefOrPoison - only compute extract/index vector elt index...

[DAG] canCreateUndefOrPoison - only compute extract/index vector elt index knownbits when not poison

We were calling computeKnownBits to determine the bounds of the element index without ensuring that it wasn't poison, meaning if we did freeze the index, isGuaranteedNotToBeUndefOrPoison would then fail as we can't call computeKnownBits through FREEZE for potentially poison values.

Fixes #92569