glsl: Handle constant expressions involving ir_binop_equal/nequal.

glsl: Handle constant expressions involving ir_binop_equal/nequal.

Constant expressions which called GLSL's equal() and notEqual()
built-ins on bvecs would hit an assertion failure; we simply forgot to
implement them for booleans.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>