[analyzer] Fix an off-by-one in evalIntegralCast()

Make sure that we do not add SymbolCast at the very boundary of
the range in which the cast would not certainly happen.

Differential Revision: http://reviews.llvm.org/D16178


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@258039 91177308-0d34-0410-b5e6-96231b3b80d8