glsl: implement switch flow control using a loop

glsl: implement switch flow control using a loop

Patch removes old variable based logic for handling a break inside
switch. Switch is put inside a loop so that existing infrastructure
for loop flow control can be used for the switch, now also dead code
elimination works properly.

Possible 'continue' call inside a switch needs now special handling
which is taken care of by detecting continue, breaking out and calling
continue for the outside loop.

v2: remove one unnecessary ir_expression (Curro)

Fixes following Piglit tests:

   fs-exec-after-break.shader_test
   fs-conditional-break.shader_test

No Piglit or es3conform regressions.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>