[InstCombine] allow splat vector folds in adjustMinMax() (retry r285732)

[InstCombine] allow splat vector folds in adjustMinMax() (retry r285732)

This was reverted at r285866 because there was a crash handling a scalar
select of vectors. I added a check for that pattern and a test case based
on the example provided in the post-commit thread for r285732.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286113 91177308-0d34-0410-b5e6-96231b3b80d8