[DAGCombiner] insert_vector_elt: Avoid building a vector twice.

This patch prevents the following combine when the input vector is used more
than once.
insert_vector_elt (build_vector elt0, ..., eltN), NewEltIdx, idx
=>
build_vector elt0, ..., NewEltIdx, ..., eltN 

The reasons are:
- Building a vector may be expensive, so try to reuse the existing part of a
  vector instead of creating a new one (think big vectors).
- elt0 to eltN now have two users instead of one. This may prevent some other
  optimizations.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187396 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8612,7 +8612,9 @@ SDValue DAGCombiner::visitINSERT_VECTOR_ELT(SDNode *N) {
   // be converted to a BUILD_VECTOR).  Fill in the Ops vector with the
   // vector elements.
   SmallVector<SDValue, 8> Ops;
-  if (InVec.getOpcode() == ISD::BUILD_VECTOR) {
+  // Do not combine these two vectors if the output vector will not replace
+  // the input vector.
+  if (InVec.getOpcode() == ISD::BUILD_VECTOR && InVec.hasOneUse()) {
     Ops.append(InVec.getNode()->op_begin(),
                InVec.getNode()->op_end());
   } else if (InVec.getOpcode() == ISD::UNDEF) {