nir: Only convert SSA values to regs when needed

If an SSA definition is used only within its block and not used for a phi, then there's no reason to lower it in nir_lower_ssa_defs_to_regs_block().

The special case for derefs is covered by the general case, so can be removed: at this point all derefs in the block are materialized (i.e. the whole deref chain is in the block) and derefs are not used in phis.