[LV] Relax assertion in fixupIVUsers (NFC).

Adjust the assertion in fixupIVUsers to only require a unique exit block
if there are any values to fix up. This enables the bring up of
multi-exit loop vectorization without requiring a scalar epilogue.

Split off as suggested from
#112138.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2823,8 +2823,6 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
   // value (the value that feeds into the phi from the loop latch).
   // We allow both, but they, obviously, have different values.
 
-  assert(OrigLoop->getUniqueExitBlock() && "Expected a single exit block");
-
   DenseMap<Value *, Value *> MissingVals;
 
   Value *EndValue = cast<PHINode>(OrigPhi->getIncomingValueForBlock(
@@ -2878,6 +2876,9 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
     }
   }
 
+  assert((MissingVals.empty() || OrigLoop->getUniqueExitBlock()) &&
+         "Expected a single exit block for escaping values");
+
   for (auto &I : MissingVals) {
     PHINode *PHI = cast<PHINode>(I.first);
     // One corner case we have to handle is two IVs "chasing" each-other,