[InstCombine] Refactor foldICmpMulConstant

This is a follow-up to 2ebfda24
(replace "if" with "else if" since the cases nuw/nsw
were meant to be handled separately).

Test plan:
1/ ninja check-llvm check-clang check-lld
2/ Bootstrapped LLVM/Clang pass tests

(cherry picked from commit d982f1e0)