[PhaseOrdering] add tests for vector select; NFC

The 1st test corresponds to a minimally optimized (mem2reg)
version of the example in:
issue #52631

The 2nd test copies an existing instcombine test with the
same pattern. If we canonicalize differently, we can miss
reducing to minimal form in a single invocation of
-instcombine, but that should not escape the normal opt
pipeline.