[DAG] canCreateUndefOrPoison - only compute shift amount knownbits when not poison

Since #93182 we can now call computeKnownBits inside getValidMaximumShiftAmount to determine the bounds of the shift amount ensuring that it wasn't poison, meaning if we did freeze the ahift amount, isGuaranteedNotToBeUndefOrPoison would then fail as we can't call computeKnownBits through FREEZE for potentially poison values.

I'm still reducing a decent test case but wanted to get the buildbot fix ASAP.