[llvm] [AMDGPU][CodeGen] Incrementally update reserved regs for SIPreAllocateWWMRegs pass in RegisterClassInfo (PR #212201)

[Matt Arsenault via llvm-commits]

================
@@ -98,6 +98,16 @@ class RegisterClassInfo {
   LLVM_ABI void runOnMachineFunction(const MachineFunction &MF,
                                      bool Rev = false);
 
+  /// allows modification of current reserved register vector
----------------
arsenm wrote:

> How about adding a restriction that you can only use this method to add registers to the reserved set, not to remove any?

We do have a potential use case for removing reserved registers. Fundamentally it shouldn't be a problem to remove from the set 

https://github.com/llvm/llvm-project/pull/212201