[IR] Make getIndexedOffsetInType return a signed result

A GEPed offset can go negative, the result of getIndexedOffsetInType
should according be a signed type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275246 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed