[llvm] [SPIRV] Add support for the SPIR-V extension SPV_KHR_bfloat16 (PR #155645)

[Dmitry Sidorov via llvm-commits]

https://github.com/MrSidims commented:

a bit of TODO (unsure if it should go in this patch or in the later patches): per SPV_KHR_bfloat extension there are limited number of instructions that can use the type. For example arithmetic instructions like FAdd or FMul can't use bfloat values, hence SPIR-V backend should either emit an error or fall back to FP32 for arithmetic. 

https://github.com/llvm/llvm-project/pull/155645