[Bug][move-compiler-v2] Wildcard patterns can be dropped

[brmataptos]

🐛 Bug

The following example (moved_var_not_simplified3.move) generates no errors in V2, as it treats the wildcard (_) assignment as a noop in stackless-bytecode generation:

module 0xc0ffee::m {
    public fun test(): u8 {
        let x = 40;
        let y = move x;
        let _ = x;
        y
    }
}

In V1, it yields the following error:

error[E06002]: use of unassigned variable
  ┌─ tests/move_check/simplifier/moved_var_not_simplified3.move:5:17
  │
4 │         let y = move x;
  │                 ------
  │                 │
  │                 The value of 'x' was previously moved here.
  │                 Suggestion: use 'copy x' to avoid the move.
5 │         let _ = x;
  │                 ^ Invalid usage of previously moved variable 'x'.

If instead of a wildcard (_) we have a variable (_z) (test case moved_var_not_simplified2.move) then V2 produces a similar error.

[brmataptos]

@wrwg points out we might even have cases where an illegal drop happens due to the wildcard. So we probably need to fix this.

[brmataptos]

OTOH, converting _ to a variable may run into trouble because it might match a tuple.