EarlyOtherwiseBranch transformation is incorrect

[tmiasko]

For details see #77163 (comment), opening an issue since PR was closed:

pub enum E<'a> {
    Empty,
    Some(&'a E<'a>),
}

fn f(e: &E) -> u32 {
   if let E::Some(E::Some(_)) = e { 1 } else { 2 }
}

fn main() {
   println!("{:?}", f(&E::Empty));
}

rustc b.rs -Zmir-opt-level=2 && ./b
"./b" terminated by signal SIGSEGV (Address boundary error)

[tmiasko]

While this one particular test case was fixed, the transformation still suffers from issues hinted at earlier comment:

1. It assumes that terminator always switches on a local that was assigned last in the block. The assumption is false.
2. It moves a read of a discriminant from the second block into first without ensuring that it holds the same value still.
3. In some executions it introduces a read that would not have happen otherwise and could have an undefined behaviour.
4. In some executions it skips execution of statements from one the blocks without checking if they are side-effect free.

cc @oli-obk