[llvm] [SPIR-V] Add support for the SPIR-V extension SPV_INTEL_bfloat16_conversion (PR #83443)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This commit is associated with a `users.noreply.github.com` email address. Per 
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https://github.com/llvm/llvm-project/pull/83443