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

[Justin Bogner via llvm-commits]

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@@ -1510,6 +1510,8 @@ VersionTuple Triple::getDXILVersion() const {
   if (getArch() != dxil || getOS() != ShaderModel)
     llvm_unreachable("invalid DXIL triple");
   StringRef Arch = getArchName();
+  if (Arch == "dxil")
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bogner wrote:

I think you can say `getSubArch() == NoSubArch` here instead of using string comparison

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