glsl: Fix depth unbalancing problem in if-statement flattening

glsl: Fix depth unbalancing problem in if-statement flattening

Previously, if max_depth were 1, the following code would see the
first if-statement (correctly) not get flattened, but the second
if-statement would (incorrectly) get flattened:

void main()
{
    if (a)
        gl_Position = vec4(0);

    if (b)
        gl_Position = vec4(1);
}

This is because the visit_leave(ir_if*) method would not decrement the
depth before returning on the first if-statement.

NOTE: This is a candidate for the 7.10 and 7.11 branches.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>