radv/nir: call opt_remove_phis after trivial continues.

radv/nir: call opt_remove_phis after trivial continues.

With the shaders in the ssao demo, the nir_opt_if wasn't
working properly without this, after this the if gets optimised
so that loop unrolling gets called.

(loop unrolling fails due to instruction count, but at least
it gets to do that.)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Cc: "17.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>