The parameters[i] is our inlined variables representing the
parameters, so they are always ir_var_auto. Walk the signature params
in handling "out" values like we do for "in" values to find the mode.
Fixes (with the previous 2 commits):
glsl1-function call with in, out params
glsl1-function call with inout params
It would be easy to miss an entry either of the two visitors involved
that would result in trying to ir->remove() the call to remove it from
the instruction stream when really it's part of an expression tree
that wasn't flattened.
