[SCEV] Combine two predicates into one; NFC

[SCEV] Combine two predicates into one; NFC

Both `loopHasNoSideEffects` and `loopHasNoAbnormalExits` involve walking
the loop and maintaining similar sorts of caches.  This commit changes
SCEV to compute both the predicates via a single walk, and maintain a
single cache instead of two.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282375 91177308-0d34-0410-b5e6-96231b3b80d8