Avoid recomputing an inserted instruction's SlotIndex.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175597 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/PHIElimination.cpp b/lib/CodeGen/PHIElimination.cpp
index 37e2ee1..061c385 100644
--- a/lib/CodeGen/PHIElimination.cpp
+++ b/lib/CodeGen/PHIElimination.cpp
@@ -306,10 +306,9 @@
   // Update LiveIntervals for the new copy or implicit def.
   if (LIS) {
     MachineInstr *NewInstr = prior(AfterPHIsIt);
-    LIS->InsertMachineInstrInMaps(NewInstr);
+    SlotIndex DestCopyIndex = LIS->InsertMachineInstrInMaps(NewInstr);
 
     SlotIndex MBBStartIndex = LIS->getMBBStartIdx(&MBB);
-    SlotIndex DestCopyIndex = LIS->getInstructionIndex(NewInstr);
     if (IncomingReg) {
       // Add the region from the beginning of MBB to the copy instruction to
       // IncomingReg's live interval.