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

[S. Bharadwaj Yadavalli via llvm-commits]

https://github.com/bharadwajy approved this pull request.

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.

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