"ref" in guarded patterns trips LLVM asserts

[bblum]

This crashes if you try to compile it:

fn main() {
    let x = some(unsafe::exclusive(false));
    match move x {
        some(ref z) if z.with(|b| *b) => {
            do z.with |b| { assert *b; }
        },
        _ => fail
    }
}

Related #2329, #3255

[nikomatsakis]

problem is that there is apparently an entirely separate code path for making the pattern bindings in the case of guard failures (inlined into compile_submatch) vs entering the arm itself (make_pattern_bindings). these two paths should be consolidated.

[nikomatsakis]

I suspect it also copies twice. This seems... non-ideal. It'd be good if we could re-use the copy we did for the guard in the arm.