Monomorphisation does not variable-ize size expressions properly

[athas]

def test (a: i64) (b: i64) (f: [a*b]f32 -> [a*b]f32) (g: [a*b]f32 -> [a*b]f32) =
  g (f (replicate (a*b) 0))

entry main a b =
  let h = test a b
  in h reverse reverse

Monomorphisation produces references to variables that do not exist, with resulting explosive effects later in the compiler.