[llvm] [DirectX] Fix DXIL part header version encoding (PR #91506)

[Xiang Li via llvm-commits]

python3kgae wrote:

> Just a reminder to make sure the sentence
> 
> "Also always normalize target triple in llc to get DXIL version from subArch."
> 
> is edited out of the "Squash and Merge" commit message.
Good catch.
Updated.

https://github.com/llvm/llvm-project/pull/91506