[InstCombine] add vector splat tests for foldLogOpOfMaskedICmps(); NFC

There's a substantial pile of scalar tests for transforms that
depend on this code, but zero vector coverage. This patch adds
a vector test next to the first scalar test in each file that
is affected by foldLogOpOfMaskedICmps.

The code that handles these transforms is artificially limited
from working with vector splat constants.