[InstCombine] allow icmp (shr/shl) folds for vectors

These 2 helper functions were already using APInt internally, so just
change the API and caller to allow folds for splats. The scalar
regression tests look quite thorough, so I just added a couple of
tests to prove that vectors are handled too.

These folds should be grouped with the other cmp+shift folds though.
That can be an NFC follow-up.

llvm-svn: 281663