[AMDGPU] Move RPT::getLiveRegs() check under EXPENSIVE_CHECKS

[rampitec]

Summary

This is too expensive even for debug builds. It doubles
scheduling time if enabled.

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

if (LiveInIt != MBBLiveIns.end()) {
     auto LiveIn = std::move(LiveInIt->second);
     RPTracker.reset(*MBB->begin(), &LiveIn);
     MBBLiveIns.erase(LiveInIt);
   } else {
     auto &Rgn = Regions[CurRegion];
     I = Rgn.first;
     auto *NonDbgMI = &*skipDebugInstructionsForward(Rgn.first, Rgn.second);
     auto LRS = BBLiveInMap.lookup(NonDbgMI);
+#ifdef EXPENSIVE_CHECKS
     assert(isEqual(getLiveRegsBefore(*NonDbgMI, *LIS), LRS));
+#endif
     RPTracker.reset(*I, &LRS);
   }
 
   for ( ; ; ) {
     I = RPTracker.getNext();
 
     if (Regions[CurRegion].first == I) {
       LiveIns[CurRegion] = RPTracker.getLiveRegs();