[SCEV] Re-enable "Cache results of computeExitLimit"

The patch rL309080 was reverted because it did not clean up the cache on "forgetValue"
method call. This patch re-enables this change, adds the missing check and introduces
two new unit tests that make sure that the cache is cleaned properly.

Differential Revision: https://reviews.llvm.org/D36087

llvm-svn: 309925