[DAGCombiner] match inverted/swapped patterns for vselect of mask of signbit

This was noted as a follow-up to D113212 / D113426:
4fc1fc40
7e30404c
11522cfc

https://alive2.llvm.org/ce/z/e4o96b

The canonicalization rules for these IR patterns are complicated,
and we were not matching the expected forms in 2 out of the 3
cases. We can make codegen more robust by matching the swapped
forms (and that will also work if these patterns are created late).