EarlyOtherwiseBranch introduces access to a dead local variable

#![feature(custom_mir, core_intrinsics)]
use std::intrinsics::mir::*;
#[custom_mir(dialect = "runtime")]
fn main() {
    mir! {
        let a: u64;
        let b: u64;
        { a = 1; StorageDead(b); match a { 0 => bb1, _ => bb3 }}
        bb1 = { match b { 0 => bb1, _ => bb3 } }
        bb2 = { Goto(bb4) }
        bb3 = { Goto(bb4) }
        bb4 = { Return() }
    }
}

rustc -Zmir-opt-level=0 -Zmir-enable-passes=+EarlyOtherwiseBranch a.rs -Zunpretty=mir

fn main() -> () {
    let mut _0: ();
    let mut _1: u64;
    let mut _2: u64;
    let mut _3: bool;
    let mut _4: bool;

    bb0: {
        _1 = const 1_u64;
        StorageDead(_2);
        _3 = Ne(copy _1, copy _2); // undefined behavior: _2 is dead
        switchInt(move _3) -> [0: bb3, otherwise: bb2];
    }

    bb1: {
        _4 = Ne(copy _2, copy _2);
        switchInt(move _4) -> [0: bb4, otherwise: bb2];
    }

    bb2: {
        return;
    }

    bb3: {
        switchInt(copy _1) -> [0: bb1, otherwise: bb2];
    }

    bb4: {
        switchInt(copy _2) -> [0: bb1, otherwise: bb2];
    }
}

Related to #159591. Pending fix appears to be quite specific to that issue, and doesn't resolve the one here.

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (9f36de775 2026-07-19)
binary: rustc
commit-hash: 9f36de775bc636c8e88c31a173c2bcb6995956a0
commit-date: 2026-07-19
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Reproduce the issue with the supplied custom MIR example and rustc command, then trace the EarlyOtherwiseBranch MIR optimization pass. Add regression coverage for this case and verify that the emitted MIR no longer accesses the dead local variable.