[Xtensa] Fix reg scavenging in XtensaRegisterInfo

Backward register scavenging (the only and default option now)
requires supportsBackwardScavenger() == true.

Author: maciej-czekaj

llvm/lib/Target/Xtensa/XtensaRegisterInfo.h

@@ -43,6 +43,10 @@ struct XtensaRegisterInfo : public XtensaGenRegisterInfo {
   bool trackLivenessAfterRegAlloc(const MachineFunction &) const override {
     return true;
   }
+  
+  bool supportsBackwardScavenger() const override {
+    return true;
+  }
 
   const uint16_t *
   getCalleeSavedRegs(const MachineFunction *MF = 0) const override;