[InstCombine] canRewriteGEPAsOffset - don't dereference a dyn_cast<>. NFCI.

We know V is a IntToPtrInst or PtrToIntInst type so we know its a CastInst - so use cast<> directly.

Prevents clang static analyzer warning that we could deference a null pointer.

Author: RKSimon

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -552,7 +552,7 @@ static bool canRewriteGEPAsOffset(Value *Start, Value *Base,
         return false;
 
       if (isa<IntToPtrInst>(V) || isa<PtrToIntInst>(V)) {
-        auto *CI = dyn_cast<CastInst>(V);
+        auto *CI = cast<CastInst>(V);
         if (!CI->isNoopCast(DL))
           return false;