glsl: Fix copy-paste error in constant_expression_value(ir_binop_nequal)

The implementation of ir_binop_nequal in constant_expression_value()
appears to have been copy-and-pasted from the implementation of
ir_binop_equal, but with all instances of '==' changed to '!='.  This
is correct except for one minor flaw: one of those '==' operators was
in an assertion checking that the types of the two arguments were
equal.  That one needs to stay an '=='.

Fixes piglit tests {fs,vs}-inline-notequal.

src/glsl/ir_constant_expression.cpp

@@ -719,7 +719,7 @@ ir_expression::constant_expression_value()
       }
       break;
    case ir_binop_nequal:
-      assert(op[0]->type != op[1]->type);
+      assert(op[0]->type == op[1]->type);
       for (unsigned c = 0; c < components; c++) {
 	 switch (op[0]->type->base_type) {
 	 case GLSL_TYPE_UINT: