[InstCombine] use m_APInt to allow icmp eq (mul X, C1), C2 folds for splat constant vectors

This concludes the splat vector enhancements for foldICmpEqualityWithConstant().
Other commits in this series:
https://reviews.llvm.org/rL277762
https://reviews.llvm.org/rL277752
https://reviews.llvm.org/rL277738
https://reviews.llvm.org/rL277731
https://reviews.llvm.org/rL277659
https://reviews.llvm.org/rL277638
https://reviews.llvm.org/rL277629

llvm-svn: 277779