Fix codegen of Memory-repr enum payload accesses

[LegNeato]

Fixes #615.

After the nightly-2026-05-22 upgrade, TryFromIntError is no longer a ZST, so Result<u32, TryFromIntError> is laid out as BackendRepr::Memory instead of ScalarPair. Multi-variant enums with this layout were translated to a tag-only OpTypeStruct, so payload field accesses couldn't be recovered into a valid OpAccessChain.

The SPIR-V type now also includes each variant's non-ZST payload fields at their absolute offsets, so payload accesses work.

[nazar-pc]

Can confirm that this one works

[tuguzT]

Yes, this PR fixes compilation issue, but requires usage of OpCapability Int8 to be actually compiled (for some unknown reason); without capability it produces such message:

  error: `u8` type used without `OpCapability Int8`
      |
      = note: used by `core::result::Result<usize, core::num::TryFromIntError>`
  note: used from within `<usize as core::convert::TryFrom<u32>>::try_from`
     --> <***\.rustup\toolchains\nightly-2026-05-22-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\convert\num.rs>:463:5
      |
  463 |     rev!(impl_try_from_unbounded, usize => u32);
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  note: called by `<u32 as core::convert::TryInto<usize>>::try_into`
     --> <***\.rustup\toolchains\nightly-2026-05-22-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\convert\mod.rs>:820:9
      |
  820 |         U::try_from(self)
      |         ^^^^^^^^^^^^^^^^^

I guess this is because of blanket implementation could not be inlined (or something similar to that)?

[nazar-pc]

Not in my case, I guess it depends on the code

[LegNeato]

Hmmm, yeah it depends on the code. TryFromIntError is actually a u8 under the covers. This has come up before in a different context, can't remember what we did so I'll poke around.

[LegNeato]

Ok, we don't currently handle this automatically in any way and just punt to the user AFAICT. That's probably correct, it would be weird to promote to u32 and also weird to add a capability silently. We also can't DCE within a type defintion.