Slicing keeps export functions that are called by kept functions

[vbgl]

The following program does not compile, even with -slice good:

/* On x86-64, c (4th argument) and n (2nd operand to shift)
 * must be allocated to the RCX register,
 * so this function cannot be compiled */
export
fn bad(reg u64 x y z c) -> reg u64 {
  reg u64 n;
  n = x;
  y <<= n & 63;
  x = y;
  x |= z;
  x |= c;
  return x;
}

// This function is fine, as c can be moved to an other free register
export
fn good(reg u64 x y z c) -> reg u64 {
  c = c;
  #[inline]
  x = bad(x, y, z, c);
  return x;
}