BasicAA: fix bug where we would return partialalias instead of noalias

My fix is conservative and will make us return may-alias instead.

The test case is:
check(gep(x, 0), n, gep(x, n), -1)  with  n == sizeof(x)

Here, the first value accesses the whole object, but the second access
doesn't access anything. The semantics of -1 is read until the end of the
object, which in this case means read nothing.

No test case, since isn't trivial to exploit this one, but I've proved it correct.

llvm-svn: 317680