[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

@@ -119,7 +119,8 @@ class MachineInstr
     Disjoint = 1 << 19,      // Each bit is zero in at least one of the inputs.
     NoUSWrap = 1 << 20,      // Instruction supports geps
                              // no unsigned signed wrap.
-    SameSign = 1 << 21       // Both operands have the same sign.
+    SameSign = 1 << 21,      // Both operands have the same sign.
+    LRSplit = 1 << 22        // Instruction for live range split.
   };
 
 private:

llvm/lib/CodeGen/SplitKit.cpp

@@ -533,6 +533,7 @@ 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();
@@ -552,6 +553,7 @@ 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();
   }