[llvm] [AArch64] Replace uses of AArch64::NoRegister with Register() or isValid() NFC (PR #220227)

[Craig Topper via llvm-commits]

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@@ -121,8 +121,8 @@ enum LiveRegs : uint8_t {
 /// Holds the virtual registers live physical registers have been saved to.
 struct PhysRegSave {
   LiveRegs PhysLiveRegs;
-  Register StatusFlags = AArch64::NoRegister;
-  Register X0Save = AArch64::NoRegister;
+  Register StatusFlags = Register();
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topperc wrote:

`Register StatusFlags;` should call the default constructor;

https://github.com/llvm/llvm-project/pull/220227