For builtin functions and operators returning bool, added generated
tests that use the return values in if statements like this:

if (a < b)
  gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
else
  gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);

in addition to the existing tests that convert the return values to
floats, like this:

bool result = a < b;
gl_FragColor = vec4(result, 0.0, 0.0, 0.0);

Since implementations may produce substantially different GPU code for
these two cases.

Reviewed-by: Chad Versace <chad@chad-versace.us>