Fixed logic error in BasicConstraintManager pointed out by Zhongxing Xu.

For checking if a symbol >= value, we need to check if symbol == value || symbol
> value. When checking symbol > value and we know that symbol != value, the path
is infeasible only if value == maximum integer.

For checking if a symbol <= value, we need to check if symbol == value || symbol
< value. When checking symbol < value and we know that symbol != value, the path
is infeasible only if value == minimum integer.

Updated test case exercising this logic: we only prune paths if the values are
unsigned.

llvm-svn: 56354