nir,zink: Make lower_discard_if a common pass
This pass (lowering discard_if to control flow and unconditional discard) was originally written for Zink, but is useful for hardware that lacks conditional discard instructions like AGX. In theory AGX could implement a conditional discard with CSEL, but the vendor compiler uses a lowering like this one. Since I like not writing code, I'd like to use the pass that's already in tree.