improve `simd_select` error message when used with invalid mask type
followup to https://github.com/rust-lang/rust/pull/137828
This PR improves the error message for an invalid `simd_select` mask type, and adds testing for `simd_scatter` and `simd_gather` being used with invalid mask types.
the `simd_masked_load` and `simd_masked_store` intrinsics already generated a better error message:
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| src | ||
| Cargo.toml | ||
| messages.ftl | ||
| README.md | ||
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.