[DAGCombiner][x86] scalarize splatted vector FP ops

There are a variety of vector patterns that may be profitably reduced to a
scalar op when scalar ops are performed using a subset (typically, the
first lane) of the vector register file.

For x86, this is true for float/double ops and element 0 because
insert/extract is just a sub-register rename.

Other targets should likely enable the hook in a similar way.

Differential Revision: https://reviews.llvm.org/D60150

llvm-svn: 357760