[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