Fix SCEV forgetMemoizedResults should search and destroy backedge exprs.

Fixes PR15570: SEGV: SCEV back-edge info invalid after dead code removal.

Indvars creates a SCEV expression for the loop's back edge taken
count, then determines that the comparison is always true and
removes it.

When loop-unroll asks for the expression, it contains a NULL
SCEVUnknkown (as a CallbackVH).

forgetMemoizedResults should invalidate the loop back edges expression.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177986 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -338,6 +338,10 @@ namespace llvm {
       /// getMax - Get the max backedge taken count for the loop.
       const SCEV *getMax(ScalarEvolution *SE) const;
 
+      /// Return true if any backedge taken count expressions refer to the given
+      /// subexpression.
+      bool hasOperand(const SCEV *S, ScalarEvolution *SE) const;
+
       /// clear - Invalidate this result and free associated memory.
       void clear();
     };