[SelectionDAG] Add MaskedValueIsZero check to allow folding of zero extended...

[SelectionDAG] Add MaskedValueIsZero check to allow folding of zero extended variables we know are safe to extend (#85573)

Add ones for every high bit that will cleared.

This will allow us to evaluate variables that have their bits known to
see if they have no risk of overflow despite the shift amount being
greater than the difference between the two types.