Make llvm.objectsize more conservative with null

In non-zero address spaces, we were reporting that an object at `null`
always occupies zero bytes. This is incorrect in many cases, so just
return `unknown` in those cases for now.

Differential Revision: https://reviews.llvm.org/D48860


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336611 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed