Add missing test for GEP + bitcast transformation

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188529 91177308-0d34-0410-b5e6-96231b3b80d8
Re-disabling C4291 warnings for MSVC because AttributeList.h requires it.  This was accidentally removed in r187279.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188530 91177308-0d34-0410-b5e6-96231b3b80d8