Add assembly test for `-Zreg-struct-return` option
r? `@tgross35`
As discussed in rust-lang/rust#145309 with `@tgross35` and `@ojeda,` I added assembly tests for the `-Zreg-struct-return` option verifying that it changes the ABI from hidden pointer to register-return on x86_32.
The test covers:
- Direct struct construction, showing register return vs hidden pointer
- External function calls returning structs, showing ABI mismatch handling
Different memory layouts affect ABI mismatch handling, but register returns use the same register allocation regardless of struct field layout (apart from the fact that they use smaller registers for smaller structs, of course).
[Here](https://godbolt.org/z/dcW6rnMG3) is a compiler explorer with 2 examples. Let me know if there is anything more I could add. Since register returns only happen for structs up to the size of 2 registers, I figured testing the pivot value (8 bytes) would be most critical.
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