LoopVectorizer: Don't assert on the absence of induction variables

A computable loop exit count does not imply the presence of an induction
variable. Scalar evolution can return a value for an infinite loop.

Fixes PR15926.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181495 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2523,7 +2523,8 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
 
   if (!Induction) {
     DEBUG(dbgs() << "LV: Did not find one integer induction var.\n");
-    assert(getInductionVars()->size() && "No induction variables");
+    if (Inductions.empty())
+      return false;
   }
 
   return true;