[SPIRV] Enable `bfloat16` arithmetic (#166031)
Enable the `SPV_INTEL_bfloat16_arithmetic` extension, which allows arithmetic, relational and `OpExtInst` instructions to take `bfloat16` arguments. This patch only adds support to arithmetic and relational ops. The extension itself is rather fresh, but `bfloat16` is ubiquitous at this point and not supporting these ops is limiting.
