glsl/ast: Generate a more compact expression to disable execution of default case

glsl/ast: Generate a more compact expression to disable execution of default case

Instead of generating a sequence like:

    run_default = true;
    if (i == 3) // some label that appears after default
        run_default = false;
    if (i == 4) // some label that appears after default
        run_default = false;
    ...
    if (run_default) {
        ...
    }

generate something like:

    run_default = !((i == 3) || (i == 4) || ...);
    if (run_default) {
        ...
    }

This eliminates one use of conditional assignment, and it enables the
elimination of another.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>