[llvm] [TableGen][GISel] Learn to import patterns with physreg defs (PR #120343)

[Matt Arsenault via llvm-commits]

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@@ -283,6 +283,8 @@ class RegisterClass<string namespace, list<ValueType> regTypes, int alignment,
   // value means copying is extremely expensive or impossible.
   int CopyCost = 1;
 
+  RegisterClass CrossCopyRegClass = ?;
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arsenm wrote:

Needs documentation. Also this doesn't seem like the right terminology. It's more of a virtual register class to use for copies with an unallocatable physical register, not a general cross copy. Maybe AllocatableRegClass?

https://github.com/llvm/llvm-project/pull/120343