fixed incorrect packed_int3 conversion generated for Metal

[39ali]

Connections
#9091

Description
translated MSL now correctly unpacks packed_type3 before using it with other unpacked types.

Testing
added a test that test against this issue

Squash or Rebase?
squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[ErichDonGubler]

question: This double cast seems incorrect, though I suspect it's harmless, but I'm not sure. @teoxoy or @jimblandy, do you know? It's also suspect that this is already being emitted in some cases, but not others. Perhaps we need to plumb a need to cast somewhere else, rather than doing it in multiple places?

[39ali]

after inspecting the metalIR this will get optimized away, in fact the compiler will handle most of the conversion between packed and unpacked, and it looks like the only issue is with the bit-cast from packed type to unpacked, i'll update the code to handle that

[ErichDonGubler]

I'll take on review. This broadly looks like it improves things, but I have questions about the approach WRT double casts.

[ErichDonGubler]

Noice! Much simpler. 👍🏻