[CodeGen] Introduce MI flag for Live Range split instructions

For some targets, it is required to identify the COPY instruction
corresponds to the RA inserted live range split. Adding the new
flag `MachineInstr::LRSplit` to serve the purpose.

Author: cdevadas

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -123,8 +123,9 @@ class MachineInstr
     NoUSWrap = 1 << 20,      // Instruction supports geps
                              // no unsigned signed wrap.
     SameSign = 1 << 21,      // Both operands have the same sign.
-    InBounds = 1 << 22       // Pointer arithmetic remains inbounds.
+    InBounds = 1 << 22,      // Pointer arithmetic remains inbounds.
                              // Implies NoUSWrap.
+    LRSplit = 1 << 23        // Instruction for live range split.
   };
 
 private:

llvm/lib/CodeGen/SplitKit.cpp

@@ -531,6 +531,7 @@ SlotIndex SplitEditor::buildSingleSubRegCopy(
               | getInternalReadRegState(!FirstCopy), SubIdx)
       .addReg(FromReg, 0, SubIdx);
 
+  CopyMI->setFlag(MachineInstr::LRSplit);
   SlotIndexes &Indexes = *LIS.getSlotIndexes();
   if (FirstCopy) {
     Def = Indexes.insertMachineInstrInMaps(*CopyMI, Late).getRegSlot();
@@ -550,6 +551,7 @@ SlotIndex SplitEditor::buildCopy(Register FromReg, Register ToReg,
     // The full vreg is copied.
     MachineInstr *CopyMI =
         BuildMI(MBB, InsertBefore, DebugLoc(), Desc, ToReg).addReg(FromReg);
+    CopyMI->setFlag(MachineInstr::LRSplit);
     return Indexes.insertMachineInstrInMaps(*CopyMI, Late).getRegSlot();
   }