[LV] Use getUniqueLatchExitBlock in createVectorLoopSkeleton (NFC).

Use getUniqueLatchExitBlock instead of getUniqueExitBlock in preparation
for multi-exit vectorization *without* requiring a scalar epilogue.

Split off as suggested from
#112138

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2600,9 +2600,9 @@ BasicBlock *InnerLoopVectorizer::emitMemRuntimeChecks(BasicBlock *Bypass) {
 void InnerLoopVectorizer::createVectorLoopSkeleton(StringRef Prefix) {
   LoopVectorPreHeader = OrigLoop->getLoopPreheader();
   assert(LoopVectorPreHeader && "Invalid loop structure");
-  assert((OrigLoop->getUniqueExitBlock() ||
+  assert((OrigLoop->getUniqueLatchExitBlock() ||
           Cost->requiresScalarEpilogue(VF.isVector())) &&
-         "multiple exit loop without required epilogue?");
+         "loops not exiting via the latch without required epilogue?");
 
   LoopMiddleBlock =
       SplitBlock(LoopVectorPreHeader, LoopVectorPreHeader->getTerminator(), DT,