[MachineCopyPropagation] Fix comment. NFC

[mcrosier]

LGTM.

lib/CodeGen/MachineCopyPropagation.cpp

if (MI->isCopy()) {
 
           MI->eraseFromParent();
           Changed = true;
           ++NumDeletes;
           continue;
         }
       }
 
-      // If Src is defined by a previous copy, it cannot be eliminated.
+      // If Src is defined by a previous copy, the previous copy cannot be
+      // eliminated.
       for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
         CI = CopyMap.find(*AI);
         if (CI != CopyMap.end()) {
           DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
           MaybeDeadCopies.remove(CI->second);
         }
       }
 
       DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
 
       // Copy is now a candidate for deletion.
       MaybeDeadCopies.insert(MI);
 
-      // If 'Src' is previously source of another copy, then this earlier copy's
+      // If 'Def' is previously source of another copy, then this earlier copy's
       // source is no longer available. e.g.
       // %xmm9<def> = copy %xmm2
       // ...
       // %xmm2<def> = copy %xmm0
       // ...
       // %xmm2<def> = copy %xmm9
       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);