Update comment

Author: sdesmalen-arm

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1526,10 +1526,11 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
 
     // In a situation like the following:
     //
-    //    undef %2.subreg:reg = INST %1:reg         ; DefMI (rematerializable),
-    //                                              ; DefSubIdx = subreg
-    //    %3:reg = COPY %2                          ; SrcIdx = DstIdx = 0
-    //    .... = SOMEINSTR %3:reg, %2
+    //    undef %2.subreg:reg = INST %1:reg    ; DefMI (rematerializable),
+    //                                         ; Defines only some of lanes,
+    //                                         ; so DefSubIdx = NewIdx = subreg
+    //    %3:reg = COPY %2                     ; Copy full reg
+    //    .... = SOMEINSTR %3:reg              ; Use full reg
     //
     // there are no subranges for %3 so after rematerialization we need
     // to explicitly create them. Undefined subranges are removed later on.