[DAGCombiner] scale repeated FP divisor by splat factor

If we have a vector FP division with a splatted divisor, use the existing transform
that converts 'x/y' into 'x * (1.0/y)' to allow more conversions. This can then
potentially be converted into a scalar FP division by existing combines (rL358984)
as seen in the tests here.

That can be a potentially big perf difference if scalar fdiv has better timing
(including avoiding possible frequency throttling for vector ops).

Differential Revision: https://reviews.llvm.org/D61028

llvm-svn: 359147