[SCEV][NFC] Strengthen safety assert in SCEV expander

[mkazantsev]

Summary

This patch enhances assert that checks that it is safe to expand a particular SCEV
in SCEVExpander with one more condition: it checks that we do not try to expand
something that is not available in the block of insertion point. For example, that we
don't try to expand a Phi to its loop's preheader.

[mkazantsev]

Split to 2 NFC changes

lib/Analysis/ScalarEvolutionExpander.cpp

if (SE.isLoopInvariant(S, L)) {
       break;
     }
 
   // Check to see if we already expanded this here.
   auto I = InsertedExpressions.find(std::make_pair(S, InsertPt));
   if (I != InsertedExpressions.end())
     return I->second;
 
-  assert(isSafeToExpand(S, SE) && "Attempt to expand something unsafe!");
+  assert(isSafeToExpandAt(S, InsertPt, SE) &&
+         "Attempt to expand something unsafe at the specified point!");
   SCEVInsertPointGuard Guard(Builder, this);
   Builder.SetInsertPoint(InsertPt);
 
   // Expand the expression into instructions.
   ScalarEvolution::ValueOffsetPair VO = FindValueInExprValueMap(S, InsertPt);
   Value *V = VO.first;
 
   if (!V)