[NFC][CodeGen][UBSan] EmitCheckedInBoundsGEP(): refactor EmitGEPOffsetInBytes() helper

It shouldn't really be inlined into the EmitCheckedInBoundsGEP().
Refactoring it beforehand will make follow-up changes more obvious.

This was originally part of https://reviews.llvm.org/D67122

llvm-svn: 371207