Correctly handle terminators in inline assembly

[XAMPPRocky]

Currently the following assembly code causes a ConsumerError(MismatchedTerminator) ICE.

Code

// build-pass

use spirv_std as _;

#[spirv(any_hit_khr)]
pub fn main() {
    unsafe {
        asm!(r#"OpExtension "SPV_KHR_ray_tracing""#);
        asm!("OpCapability RayTracingKHR", "OpTerminateRayKHR");
    }
}

Error

thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: ConsumerError(MismatchedTerminator)', crates/rustc_codegen_spirv/src/link.rs:423:58
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/EmbarkStudios/rust-gpu/issues/new

note: rustc 1.52.0-nightly (61edfd591 2021-03-20) running on x86_64-apple-darwin

note: compiler flags: -Z codegen-backend=/Users/erin.power/src/rust-gpu2/target/release/deps/librustc_codegen_spirv.dylib -Z unstable-options -Z crate-attr=no_std -Z crate-attr=feature(register_attr,asm) -Z crate-attr=register_attr(spirv) -C prefer-dynamic -C overflow-checks=off -C debug-assertions=off -C debuginfo=2 -C embed-bitcode=no --crate-type dylib

[XAMPPRocky]

Currently you can work around this by adding unused OpLabel at the end of the asm! block. Also to correctly implement terminators on the Rust side changing their type signature to -> !, we need to close #570

asm!("OpKill", "%foo = OpLabel");

[khyperia]

Fixed by #717