[llvm] [CodeGen][MRI] Introduce synthetic register classes (PR #86006)

[Pierre van Houtryve via llvm-commits]

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@@ -47,6 +47,7 @@ class MCRegisterClass {
   const int8_t CopyCost;
   const bool Allocatable;
   const bool BaseClass;
+  const bool Synthetic;
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Pierre-vh wrote:

Maybe those should be bitfields/a mask?

https://github.com/llvm/llvm-project/pull/86006