[NFC] Remove redundant code

[tomatih]

Summary

Coverity flagged an "Unchecked return value" issue.
I have investigated it and the isUndef function is constant with no arguments, only returning a value.
The function does make an assert, but it isn't relevant as next time Cond[1] is used is in the preserveCondRegFlags function which again calls isUndef on it this time using the return value.
As such the line is truly redundant and has been removed.

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

unsigned SIInstrInfo::insertBranch(MachineBasicBlock &MBB,
   }
 
   assert(TBB && Cond[0].isImm());
 
   unsigned Opcode
     = getBranchOpcode(static_cast<BranchPredicate>(Cond[0].getImm()));
 
   if (!FBB) {
-    Cond[1].isUndef();
     MachineInstr *CondBr =
       BuildMI(&MBB, DL, get(Opcode))
       .addMBB(TBB);
 
     // Copy the flags onto the implicit condition register operand.
     preserveCondRegFlags(CondBr->getOperand(1), Cond[1]);
     fixImplicitOperands(*CondBr);