[DAGCombiner] add one-use restriction to vector transform with cheap extract

[DAGCombiner] add one-use restriction to vector transform with cheap extract

We might be able to do better on the example in the test,
but in general, we should not scalarize a splatted vector
binop if there are other uses of the binop. Otherwise, we
can end up with code as we had - a scalar op that is
redundant with a vector op.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372886 91177308-0d34-0410-b5e6-96231b3b80d8