Check that patterns bind the same variables

[mtzguido]

Not a big deal, but the following code

let f o =
    match o with
    | (Some x, _)
    | (x, Some y) -> y
    | _ -> false

fails with

A.fst(6,21-6,22): (Error 230) Variable "y#6" not found

which is confusing if the match is a big one. Somewhat related to #1121.

[A-Manning]

Fixed in #1404