Fix ICE #152204/#149156: ABI check fails for unreachable code

[rynewang]

Summary

Fix ICE when ABI computation fails for monomorphized instances that never reach codegen.

Problem

#![feature(portable_simd)]
fn main() {
    if false {
        let _ = core::simd::Simd::<u8, 256>::splat(0);
    }
}

ICEs because check_instance_abi uses delayed_bug, assuming codegen will report the error. But unreachable code is monomorphized without reaching codegen.

Fix

Replace delayed_bug with emit_err to report the ABI/layout error directly.

Includes regression test.

Fixes rust-lang#152204
Fixes rust-lang#149156