[AMDGPU] Fix data race on RegisterBank initialization.

[huihuiz]

Summary

The initialization of RegisterBank needs to be done only once. The
logic of AlreadyInit has data race, use llvm::call_once instead.

This is continuing work of D73587.

[nhaehnle]

LGTM, thanks.

llvm/lib/Target/AMDGPU/AMDGPURegisterBankInfo.cpp

 }
 AMDGPURegisterBankInfo::AMDGPURegisterBankInfo(const GCNSubtarget &ST)
     : AMDGPUGenRegisterBankInfo(),
       Subtarget(ST),
       TRI(Subtarget.getRegisterInfo()),
       TII(Subtarget.getInstrInfo()) {
 
   // HACK: Until this is fully tablegen'd.
-  static bool AlreadyInit = false;
-  if (AlreadyInit)
-    return;
-
-  AlreadyInit = true;
+  static llvm::once_flag InitializeRegisterBankFlag;
 
+  static auto InitializeRegisterBankOnce = [this]() {
     assert(&getRegBank(AMDGPU::SGPRRegBankID) == &AMDGPU::SGPRRegBank &&
            &getRegBank(AMDGPU::VGPRRegBankID) == &AMDGPU::VGPRRegBank &&
            &getRegBank(AMDGPU::AGPRRegBankID) == &AMDGPU::AGPRRegBank);
+  };
+
+  llvm::call_once(InitializeRegisterBankFlag, InitializeRegisterBankOnce);
 }
 
 static bool isVectorRegisterBank(const RegisterBank &Bank) {
   unsigned BankID = Bank.getID();
   return BankID == AMDGPU::VGPRRegBankID || BankID == AMDGPU::AGPRRegBankID;
 }
 
 unsigned AMDGPURegisterBankInfo::copyCost(const RegisterBank &Dst,