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

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@371207 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang