nir/spirv/cfg: Detect switch_break after loop_break/continue

nir/spirv/cfg: Detect switch_break after loop_break/continue

While the current CFG code is valid in the case where a switch break also
happens to be a loop continue, it's a bit suboptimal.  Since hardware is
capable of handling the continue as a direct jump, it's better to use a
continue instruction when we can than to bother with all of the nasty
switch break lowering.

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