[mlir][x86vector] AVX512-BF16 Dot op (#124800)
Adds AVX512 bf16 dot-product operation and defines lowering to LLVM
intrinsics.
AVX512 intrinsic operation definition is extended with an optional
extension field that allows specifying necessary LLVM mnemonic suffix
e.g., `"bf16"` for `x86_avx512bf16_` intrinsics.
