[AMDGPU] Remove misplaced assert. (llvm#154187)

The assert that RegScavenger required for long branching is now
placed below the code to use s_add_pc64, where it is actually
used.

Author: rampitec

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -2905,7 +2905,6 @@ void SIInstrInfo::insertIndirectBranch(MachineBasicBlock &MBB,
                                        MachineBasicBlock &RestoreBB,
                                        const DebugLoc &DL, int64_t BrOffset,
                                        RegScavenger *RS) const {
-  assert(RS && "RegScavenger required for long branching");
   assert(MBB.empty() &&
          "new block should be inserted for expanding unconditional branch");
   assert(MBB.pred_size() == 1);