fix PR17635: false positive with packed structures

fix PR17635: false positive with packed structures
LLVM optimizers may widen accesses to packed structures that overflow the structure itself, but should be in bounds up to the alignment of the object

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193317 91177308-0d34-0410-b5e6-96231b3b80d8