[GVN] Don't explicitly materialize undefs from dead blocks

When materializing an available load value, do not explicitly
materialize the undef values from dead blocks. Doing so will
will force creation of a phi with an undef operand, even if there
is a dominating definition. The phi will be folded away on
subsequent GVN iterations, but by then we may have already
poisoned MDA cache slots.

Simply don't register these values in the first place, and let
SSAUpdater do its thing.