returns false in the event the computation feeding into the pointer is
unreachable, which maybe ought to be true -- but this is at least consistent
with undef->isDereferenceablePointer().) Fixes PR11825!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148671 91177308-0d34-0410-b5e6-96231b3b80d8
