[SCEV] Turn validity check in getExistingSCEV into assert (NFC).

[fhahn]

Summary

Now that we track users of SCEV expressions, we should be able to always
invalidate containing expressions.

With that, I think the case where a value gets removed but
SCEVs containing references to it should not be possible any longer.
Turn check into an assert.

This slightly reduces compile-time:

NewPM-O3: -0.27%
NewPM-ReleaseThinLTO: -0.21%
NewPM-ReleaseLTO-g: -0.26%

http://llvm-compile-time-tracker.com/compare.php?from=c3dc6b081da6ba503e67d260033f81f61eb38ea3&to=95a4a028b1f1dd0bc3d221435953b7d2c031b3d5&stat=instructions

[nikic]

Duplicate of D113349/D114260.

[nikic]

LGTM, now that D113349 is relanded. I'll abandon D114260 as the request there was to keep the assert, which is exactly what you did here.

[fhahn]

In D114633#3156405, @nikic wrote:

LGTM, now that D113349 is relanded. I'll abandon D114260 as the request there was to keep the assert, which is exactly what you did here.

Sounds good & thanks for linking the related patches, I missed them!

If I understand @mkazantsev's comment for D114260 correctly, we could still remove the check after the assert has been active for a while, to catch any potential offenders. So perhaps we could land D114260 in a month or 2?

llvm/lib/Analysis/ScalarEvolution.cpp

 }
 
 const SCEV *ScalarEvolution::getExistingSCEV(Value *V) {
   assert(isSCEVable(V->getType()) && "Value is not SCEVable!");
 
   ValueExprMapType::iterator I = ValueExprMap.find_as(V);
   if (I != ValueExprMap.end()) {
     const SCEV *S = I->second;
-    if (checkValidity(S))
-      return S;
-    eraseValueFromMap(V);
-    forgetMemoizedResults(S);
+    assert(checkValidity(S) &&
+           "existing SCEV has not been properly invalidated");
   }
   return nullptr;
 }
 
 /// Return a SCEV corresponding to -V = -1*V
 const SCEV *ScalarEvolution::getNegativeSCEV(const SCEV *V,
                                              SCEV::NoWrapFlags Flags) {
   if (const SCEVConstant *VC = dyn_cast<SCEVConstant>(V))