[SCEV] Turn check in createSimpleAffineAddRec to assertion. (NFC)

[fhahn]

Summary

Accum is guaranteed to be defined outside L (via Loop::isLoopInvariant
checks above). I think that should guarantee that the more powerful
ScalarEvolution::isLoopInvariant also determines that the value is loop
invariant.

[nikic]

LG

llvm/lib/Analysis/ScalarEvolution.cpp

const SCEV *ScalarEvolution::createSimpleAffineAddRec(PHINode *PN,
   const SCEV *StartVal = getSCEV(StartValueV);
   const SCEV *PHISCEV = getAddRecExpr(StartVal, Accum, L, Flags);
 
   ValueExprMap[SCEVCallbackVH(PN, this)] = PHISCEV;
 
   // We can add Flags to the post-inc expression only if we
   // know that it is *undefined behavior* for BEValueV to
   // overflow.
-  if (auto *BEInst = dyn_cast<Instruction>(BEValueV))
-    if (isLoopInvariant(Accum, L) && isAddRecNeverPoison(BEInst, L))
+  if (auto *BEInst = dyn_cast<Instruction>(BEValueV)) {
+    assert(isLoopInvariant(Accum, L) &&
+           "Accum is defined outside L, but is not invariant?");
+    if (isAddRecNeverPoison(BEInst, L))
       (void)getAddRecExpr(getAddExpr(StartVal, Accum), Accum, L, Flags);
+  }
 
   return PHISCEV;
 }
 
 const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
   const Loop *L = LI.getLoopFor(PN->getParent());
   if (!L || L->getHeader() != PN->getParent())
     return nullptr;