[SCEV] Use object size for globals to sharpen ranges.

The highest address the object can start is ObjSize bytes before the
end (unsigned max value). If this value is not a multiple of the
alignment, the last possible start value is the next lowest multiple
of the alignment. Note: The computations cannot overflow,
because if they would there's no possible start address for the
object.

At the moment, this is limited to GlobalVariables, because I could not
find a API similar to getObjectSize to also get the alignment of the
object. With such an API, this can be generalized to general addresses.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149483