[RegisterCoalescer] Make resolveConflicts aware of earlyclobber

[arcbbb]

Summary

Prior to this patch, it skipped the instruction defining VNI when checking if the tainted lanes are used.
In the given example, VRGATHER is an illegal instruction because its DstReg overlaps with SrcReg.

Therefore we need to check the defining instruction as well when there is an earlyclobber constraint.

[arcbbb]

update a test case to show the difference

[arcbbb]

clang-formatted

[arcbbb]

Avoid postfix increment operator.

[craig.topper]

@qcolombet, can you take a look at this?

[qcolombet]

LGTM

llvm/lib/CodeGen/RegisterCoalescer.cpp

for (unsigned i = 0, e = LR.getNumValNums(); i != e; ++i) {
 
     assert(!TaintExtent.empty() && "There should be at least one conflict.");
 
     // Now look at the instructions from VNI->def to TaintExtent (inclusive).
     MachineBasicBlock *MBB = Indexes->getMBBFromIndex(VNI->def);
     MachineBasicBlock::iterator MI = MBB->begin();
     if (!VNI->isPHIDef()) {
       MI = Indexes->getInstructionFromIndex(VNI->def);
+      if (!VNI->def.isEarlyClobber()) {
           // No need to check the instruction defining VNI for reads.
-      ++MI;
+          MI++;
+      }
     }
     assert(!SlotIndex::isSameInstr(VNI->def, TaintExtent.front().first) &&
            "Interference ends on VNI->def. Should have been handled earlier");
     MachineInstr *LastMI =
       Indexes->getInstructionFromIndex(TaintExtent.front().first);
     assert(LastMI && "Range must end at a proper instruction");
     unsigned TaintNum = 0;
     while (true) {