Invalid CLIF generated from conditional branch rewrite optimizations

[alexcrichton]

This input, fuzz generated, fails with:

$ cargo run test ../out.clif
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
     Running `/home/alex/code/wasmtime/target/debug/clif-util test ../out.clif`
[2026-05-14T13:31:21Z ERROR cranelift_filetests::concurrent] FAIL: Compilation error: Verifier errors
FAIL ../out.clif: Compilation error: Verifier errors

Caused by:
    0: Verifier errors
    1: - inst334: invalid block reference block12

1 tests
Error: 1 failure

Bisection shows #13267 as the culprit (cc @fitzgen)