Update OCL nan builtin translation to work with bfloat type (#3558)
To avoid name clash between builtins with `half` and `bfloat` return types, introduce return type postfix for
`bfloat nan` builtin in reverse translation to SPIR-V Friendly IR.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/3ce38f4ecb3da94