[SCEVAA] Enhance SCEVAAResult::alias() to handle two pointers with different pointer bases (#91453)

This patch enhances the SCEVAAResult::alias() interface to handle two
pointers with different pointer bases.

Before calling getMinusSCEV(), we firstly try to explicitly convert
these two pointers into ptrtoint expressions to do that.

Either both pointers are used with ptrtoint or neither, so we can't
end up with a ptr + int mix.