Name conflict with --preserve-all-globals

[therontarigo]

With input such as

float o(vec3 q)
{
 return min(oa(q),ob(q));
}

and --preserve-all-globals option, the name might be reused for a variable, which is not valid:

e=o(c),c+=e*o;

Found in https://github.com/laurentlb/shader-minifier/blob/master/tests/real/to_the_road_of_ribbon.expected