[DAG] computeKnownBits - Move (most) ISD::SRL handling into KnownBits::lshr

As discussed on D90527, we should be be trying to move shift handling functionality into KnownBits to avoid code duplication in SelectionDAG/GlobalISel/ValueTracking.

The refactor to use the KnownBits fixed/min/max constant helpers allows us to hit a couple of cases that we were missing before.

We still need the getValidMinimumShiftAmountConstant case as KnownBits doesn't handle per-element vector cases.