[DAGCombiner] add fold for vselect based on mask of signbit, part 2

This is the 'or' sibling for the fold added with:
D113212

https://alive2.llvm.org/ce/z/tgnp7K

Note that neither of these transforms is poison-safe,
but it does not seem to matter at this level. We have
had the scalar version of D113212 for a long time, so
this is just making optimizer behavior consistent.

We do not have the scalar version of *this* fold,
however, so that is another follow-up.