[llvm] [X86] Add missing subvector_subreg_lowering for BF16 (PR #83720)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: https://github.com/phoebewang/llvm-project/commit/7210a98061d9f6382e8ce4ca7646ec4a5c3f96e6

https://github.com/llvm/llvm-project/actions/runs/8138104765

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/83720