[InstCombine] fix bug when offsetting case values of a switch (PR31260)

We could truncate the condition and then try to fold the add into the
original condition value causing wrong case constants to be used.

Move the offset transform ahead of the truncate transform and return
after each transform, so there's no chance of getting confused values.

Fix for:
https://llvm.org/bugs/show_bug.cgi?id=31260

llvm-svn: 289442