nir/from_ssa: Don't try to read an invalid instruction

nir/from_ssa: Don't try to read an invalid instruction

Right now, the nir_instr_prev function function blindly looks up the
previous element in the exec list and casts it to an instruction even if
it's the tail sentinel.  The next commit will change this to return null if
it's the first instruction.  Making this change first avoids getting a
segfault between commits.  The only reason we never noticed is that, thanks
to the way things are laid out in nir_block, the casted instruction's type
was never parallal_copy.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>