Merging r372886:

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r372886 | spatel | 2019-09-25 08:08:33 -0700 (Wed, 25 Sep 2019) | 7 lines

[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.
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