[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. llvm-svn: 187396
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