nir/lower_atomics: Use/support SSA

nir/lower_atomics: Use/support SSA

Previously, lower_atomics was non-SSA only.  We assert-failed if the
destination of an atomic operation intrinsic was an SSA def and we used
temporary registers for computing offsets.  This commit changes both of
these behaviors.  We now use SSA values for computing offsets (so we can
optimize them) and we handle SSA destinations.  We also move the pass to
run before we go out of SSA on i965 as it now generates SSA values.

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