[Hexagon] Check isInstr() before getInstr() with SUnit.

[Xinglong]

Summary

SUnit represent a MachineInstr in post-regalloc scheduling but SDNode in pre-regalloc scheduling. when pass -enable-hexagon-sdnode-sched to Hexagon backend with -O1 and above, this may cause an assertion failed.

Fixes PR45194.

llvm/lib/Target/Hexagon/HexagonSubtarget.cpp

bool HexagonSubtarget::useAA() const {
   return false;
 }
 
 /// Perform target specific adjustments to the latency of a schedule
 /// dependency.
 void HexagonSubtarget::adjustSchedDependency(SUnit *Src, int SrcOpIdx,
                                              SUnit *Dst, int DstOpIdx,
                                              SDep &Dep) const {
-  MachineInstr *SrcInst = Src->getInstr();
-  MachineInstr *DstInst = Dst->getInstr();
   if (!Src->isInstr() || !Dst->isInstr())
     return;
 
+  MachineInstr *SrcInst = Src->getInstr();
+  MachineInstr *DstInst = Dst->getInstr();
   const HexagonInstrInfo *QII = getInstrInfo();
 
   // Instructions with .new operands have zero latency.
   SmallSet<SUnit *, 4> ExclSrc;
   SmallSet<SUnit *, 4> ExclDst;
   if (QII->canExecuteInBundle(*SrcInst, *DstInst) &&
       isBestZeroLatency(Src, Dst, QII, ExclSrc, ExclDst)) {
     Dep.setLatency(0);