[llvm] [RegAllocFast] Document the allocator state and operand phases. NFC (PR #219835)

[Nikhil Kotikalapudi via llvm-commits]

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@@ -1452,8 +1452,7 @@ void RegAllocFastImpl::findAndSortDefOperandIndexes(const MachineInstr &MI) {
   });
 }
 
-// Returns true if MO is tied and the operand it's tied to is not Undef (not
-// Undef is not the same thing as Def).
+// Returns true if MO is tied to an operand that is not undef.
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nkotikal wrote:

That makes sense, lgtm

https://github.com/llvm/llvm-project/pull/219835