[InstCombine] fix constant to be signed for signed comparisons

This bug was introduced in r269728 and is the likely cause of many stage 2 ubsan bot failures.
I'll add a test in a follow-up commit assuming this fixes things properly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269797 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed