Ensure FillRC is always initialised.

Author: paulwalker-arm

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -6288,10 +6288,9 @@ MachineInstr *AArch64InstrInfo::foldMemoryOperandImpl(
     //   LDRWui %0:sub_32<def,read-undef>, %stack.0
     //
     if (IsFill && SrcMO.getSubReg() == 0 && DstMO.isUndef()) {
-      const TargetRegisterClass *FillRC;
+      const TargetRegisterClass *FillRC = nullptr;
       switch (DstMO.getSubReg()) {
       default:
-        FillRC = nullptr;
         break;
       case AArch64::sub_32:
         if (AArch64::GPR64RegClass.hasSubClassEq(getRegClass(DstReg)))