Allow MaybeUninit in input and output of inline assembly

[taiki-e]

Fixes #1428

MaybeUninit registers have been supported since rust-lang/rust#114790.
This PR uses the same way as that used in that PR.

Tested on a crate with heavy use of MaybeUninit registers (https://github.com/taiki-e/atomic-maybe-uninit).

$ git clone https://github.com/taiki-e/atomic-maybe-uninit
$ cd atomic-maybe-uninit

# With `-Z codegen-backend=cranelift`
$ RUSTFLAGS='-Z codegen-backend=cranelift' ./tools/test.sh --tests 
+ RUSTFLAGS='-Z codegen-backend=cranelift -C panic=abort -Z panic_abort_tests' \
+ RUSTDOCFLAGS=' -C panic=abort -Z panic_abort_tests' \
+ cargo test --tests --all-features --workspace
   Compiling atomic-maybe-uninit v0.3.1 (/Users/taiki/projects/sources/taiki-e/atomic-maybe-uninit)
thread 'rustc' panicked at src/inline_asm.rs:790:45:
called `Option::unwrap()` on a `None` value

# With this PR
$ cargo=path/to/dist/cargo-clif ./tools/test.sh --tests
+ RUSTFLAGS=' -C panic=abort -Z panic_abort_tests' \
+ RUSTDOCFLAGS=' -C panic=abort -Z panic_abort_tests' \
+ path/to/dist/cargo-clif test --tests --all-features --workspace
   Compiling atomic-maybe-uninit v0.3.1 (/Users/taiki/projects/sources/taiki-e/atomic-maybe-uninit)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.03s
     Running unittests src/lib.rs (target/debug/deps/atomic_maybe_uninit-299d38e254d4f4b0)

running 192 tests
<omitted>
test result: ok. 192 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.39s

[bjorn3]

Thanks!