[Attributor] Regularly clear dependences to remove spurious ones

As dependences between abstract attributes can become stale, e.g., if
one was sufficient to imply another one at some point but it has since
been wakened to the point it is not usable for the formerly implied one.
To weed out spurious dependences, and thereby eliminate unneeded
updates, we introduce an option to determine how often the dependence
cache is cleared and recomputed during the fixpoint iteration.

Note that the initial value was determined such that we see a positive
result on our tests.

Differential Revision: https://reviews.llvm.org/D63315

llvm-svn: 370230