[DAGCombiner] refactor folds for fadd (fmul X, -2.0), Y; NFCI

The transform doesn't work if the vector constant has undef elements.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344532 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed