[llvm] [NFC][SPIRV] Add AMDGCN SPIR-V specific defaults to the BE (PR #165815)

[Alex Voicu via llvm-commits]

https://github.com/AlexVlx commented:

> nit: Isn't AMD's generator magic number `10`? 
> 
> https://github.com/KhronosGroup/SPIRV-Headers/blob/f2e4bd213104fe323a01e935df56557328d37ac8/include/spirv/spir-v.xml#L34 

Unless I am mistaken that is for the GFX side / LLPC (somewhat unfortunate that we have to diverge, but it would probably be more confusing to overlap).

https://github.com/llvm/llvm-project/pull/165815