[ScalarEvolution] Handle <= and >= in non infinite loops

Extend scalar evolution to handle >= and <= if a loop is known to be finite and the induction variable guards the condition. Specifically, with these assumptions lhs <= rhs is equivalent to lhs < rhs + 1 and lhs >= rhs to lhs > rhs -1.

In the case of lhs <= rhs, this is true since the only case these are not equivalent
is when rhs == unsigned/signed intmax, which would have resulted in an infinite loop.

In the case of lhs >= rhs, this is true since the only case these are not equivalent
is when rhs == unsigned/signed intmin, which would again have resulted in an infinite loop.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D118090