[naga msl-out] Avoid undefined behaviour due to signed integer overflow

[jamienicol]

Connections
Fixes #6666

Description
Signed integer overflow is undefined behaviour in MSL. However, signed integers are defined to be two's complement. This allows us to cast signed values to their corresponding unsigned type, perform the arithmetic on these unsigned values (which has defined overflow behaviour) then cast the result back to signed.

Care must be taken when emitting the isign polyfill, which uses metal::select(). We must ensure the -1, 0, and 1 literals used as inputs to select() have the correct width, else bitcasting the output of select() will fail due to mismatched widths.

Testing
Inspected shapshot test output. Ensure validation still succeeds.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[teoxoy]

Good stuff!

[teoxoy]

There are some edge cases that will overflow but we have to handle those differently, I will open a new issue for those.

div: T::MIN / -1 = T::MIN
rem: T::MIN % -1 = 0
abs: abs(T::MIN) = T::MIN
neg: -T::MIN = T::MIN

The results above are what the WGSL spec defines.