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

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

Differential Revision: https://reviews.llvm.org/D97232

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

@@ -461,7 +461,9 @@ void GCNScheduleDAGMILive::computeBlockPressure(const MachineBasicBlock *MBB) {
     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);
   }