Remove out of date comment. NFCI.

DAGCombiner::convertBuildVecZextToZext just requires the extractions to be sequential, they don't have to start from 0'th index.

llvm-svn: 356552

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16826,7 +16826,6 @@ SDValue DAGCombiner::reduceBuildVecToShuffle(SDNode *N) {
 // a vector zero extend and possibly an extract subvector.
 // TODO: Support sign extend or any extend?
 // TODO: Allow undef elements?
-// TODO: Don't require the extracts to start at element 0.
 SDValue DAGCombiner::convertBuildVecZextToZext(SDNode *N) {
   if (LegalOperations)
     return SDValue();