nir/spirv/cfg: Handle switches whose break block is a loop continue

nir/spirv/cfg: Handle switches whose break block is a loop continue

It is possible that the break block of a switch is actually the continue of
the loop containing the switch.  In this case, we need to identify the
break block as a continue and break out of current level of CFG handling.
If we don't, the continue portion of the loop will get handled twice, once
by following after the break and a second time by the loop handling code
handling it explicitly.

This fixes 6 of the new Vulkan CTS tests:
 - dEQP-VK.spirv_assembly.instruction.graphics.opphi.out_of_order*
 - dEQP-VK.spirv_assembly.instruction.graphics.selection_block_order.out_of_order*

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>