nir: implement the GLSL equivalent of if simplication in nir_opt_if

nir: implement the GLSL equivalent of if simplication in nir_opt_if

This pass turns:

   if (cond) {
   } else {
      do_work();
   }

into:

   if (!cond) {
      do_work();
   } else {
   }

Here's the vkpipeline-db stats (from affected shaders) on Polaris10:

Totals from affected shaders:
SGPRS: 17272 -> 17296 (0.14 %)
VGPRS: 18712 -> 18740 (0.15 %)
Spilled SGPRs: 1179 -> 1142 (-3.14 %)
Code Size: 1503364 -> 1515176 (0.79 %) bytes
Max Waves: 916 -> 911 (-0.55 %)

This pass only affects Serious Sam 2017 (Vulkan) on my side. The
stats are not really good for now. Some shaders look quite dumb
but this will be improved with further NIR passes, like ifs
combination.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>