Reapply r198478 "Fix PR18361: Invalidate LoopDispositions after LoopSimplify hoists things."

Now with a fix for PR18384: ValueHandleBase::ValueIsDeleted.

We need to invalidate SCEV's loop info when we delete a block, even if no values are hoisted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198631 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -784,6 +784,13 @@ namespace llvm {
     /// disconnect it from a def-use chain linking it to a loop.
     void forgetValue(Value *V);
 
+    /// \brief Called when the client has changed the disposition of values in
+    /// this loop.
+    ///
+    /// We don't have a way to invalidate per-loop dispositions. Clear and
+    /// recompute is simpler.
+    void forgetLoopDispositions(const Loop *L) { LoopDispositions.clear(); }
+
     /// GetMinTrailingZeros - Determine the minimum number of zero bits that S
     /// is guaranteed to end in (at every loop iteration).  It is, at the same
     /// time, the minimum number of times S is divisible by 2.  For example,