nir: Allow derefs to be used as phi sources

It is possible and valid for a pointer to be selected based on a conditional before used, and depending on the mode, those cases will result in a phi with derefs as sources.

To achieve this, we don't rematerialize derefs that are used by phis. As a consequence, when converting from SSA to regs, we may have phis that come from different blocks and are used by phis. We now convert those to regs too.

Validation was added to ensure only derefs of certain modes can be used as phi sources. No extra validation is needed for the presence of cast, any instruction that uses derefs will validate the deref-chain is complete (ending in a cast or a var).