MIR borrowck: errors unreported in unreachable arms of match expressions

[zilbuz]

Note from @pnkfelix for people looking at this bug: See important soundness issue in ariel's issue.

In the following code the borrow error in the z arm isn't reported with MIR borrowck, but it is reported with AST borrowck:

fn main() {
    let mut x = 1;
    let _x = &mut x;
    match x {
        y => println!("y: {}", y), // Error in AST and MIR
        z => println!("z: {}", z), // Error only in AST
    }
}

(compile with -Z emit-end-regions -Z borrowck-mir to use the MIR borrowck)

[mikhail-m1]

my idea of fix mikhail-m1@ff15f84, it works but doesn't finished