[CodeGen][NFC] Add comment to `getCSRFirstUseCost`

[lkail]

Summary

From usage in RAGreedy::initializeCSRCost and AArch64RegisterInfo::getCSRFirstUseCost, this value is set assuming MachineBlockFrequencyInfo::getEntryFreq() is 1<<14. Add comment to clarify it so that other target can override it accordingly.

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

public:
   /// don't benefit from reversing the order.
   /// (2) Bottom-up allocation could result in poor evicition decisions on some
   /// targets affecting the performance of compiled code.
   /// (3) Bottom-up allocation is no longer guaranteed to optimally color.
   virtual bool reverseLocalAssignment() const { return false; }
 
   /// Allow the target to override the cost of using a callee-saved register for
   /// the first time. Default value of 0 means we will use a callee-saved
-  /// register if it is available.
+  /// register if it is available. This value is set assuming entry block
+  /// frequency is 1<<14.
   virtual unsigned getCSRFirstUseCost() const { return 0; }
 
   /// Returns true if the target requires (and can make use of) the register
   /// scavenger.
   virtual bool requiresRegisterScavenging(const MachineFunction &MF) const {
     return false;
   }