SCT checker too coarse with stack arguments

[vbgl]

This is a regression introduced in #847. The following program is no longer accepted by the SCT checker.

inline
fn reset(stack u64 t) {
  [t] = 0;
}

export
fn main(reg u64 x) {
  _ = #init_msf();
  stack u64 s;
  s = x;
  reset(s);
}