[llvm] [X86] Report an error for FP environment accesses that are not 256 bits (PR #228841)

[Doha Nam via llvm-commits]

waroad wrote:

I have read the LLVM AI Tool Use Policy, the Code-Review Policy and the Developer Policy. This patch was prepared with AI assistance (Claude); this is noted in the PR description via the Co-Authored-By trailer. I reviewed the change and the added test, and checked that the X86 and Generic CodeGen lit tests pass.

https://github.com/llvm/llvm-project/pull/228841