nir: Only convert SSA values to regs when needed

nir: Only convert SSA values to regs when needed

If the SSA def produced by this instruction is only in the block in
which it is defined and is not used by ifs or phis, then we don't have
a reason to convert it to a register 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.

v2: Fix wrong check for if_uses.  If there's such an use, the def is
    not "local_to_block".  (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>