Create a getelementptr instead of sub expr for ValueOffsetPair if the

Create a getelementptr instead of sub expr for ValueOffsetPair if the
value is a pointer.

This patch is to fix PR30213. When expanding an expr based on ValueOffsetPair,
if the value is of pointer type, we can only create a getelementptr instead
of sub expr.

Differential Revision: https://reviews.llvm.org/D24088

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281439 91177308-0d34-0410-b5e6-96231b3b80d8