[Xtensa] Implement branch relaxation.

Author: andreisfr

llvm/lib/Target/Xtensa/XtensaAsmPrinter.cpp

@@ -69,6 +69,9 @@ void XtensaAsmPrinter::emitMachineConstantPoolValue(
     const BlockAddress *BA =
         cast<XtensaConstantPoolConstant>(ACPV)->getBlockAddress();
     MCSym = GetBlockAddressSymbol(BA);
+  } else if (ACPV->isMachineBasicBlock()) {
+    const MachineBasicBlock *MBB = cast<XtensaConstantPoolMBB>(ACPV)->getMBB();
+    MCSym = MBB->getSymbol();
   } else if (ACPV->isJumpTable()) {
     unsigned Idx = cast<XtensaConstantPoolJumpTable>(ACPV)->getIndex();
     MCSym = this->GetJTISymbol(Idx, false);