[LV] Prefer DenseMap::lookup over find (NFC) (#141809)

Apart from the stylistic improvement, lookup has the nice property of
returning a default-constructed object on failure-to-find, while find
returns the end iterator, which cannot be dereferenced.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 05fd87e..fc8ebeb 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2548,9 +2548,9 @@
     return C->getValue();
   if (auto *U = dyn_cast<SCEVUnknown>(Step))
     return U->getValue();
-  auto I = ExpandedSCEVs.find(Step);
-  assert(I != ExpandedSCEVs.end() && "SCEV must be expanded at this point");
-  return I->second;
+  Value *V = ExpandedSCEVs.lookup(Step);
+  assert(V && "SCEV must be expanded at this point");
+  return V;
 }
 
 /// Knowing that loop \p L executes a single vector iteration, add instructions
@@ -9860,8 +9860,8 @@
     auto *ExpandR = dyn_cast<VPExpandSCEVRecipe>(&R);
     if (!ExpandR)
       continue;
-    auto *ExpandedVal =
-        Plan.getOrAddLiveIn(ExpandedSCEVs.find(ExpandR->getSCEV())->second);
+    VPValue *ExpandedVal =
+        Plan.getOrAddLiveIn(ExpandedSCEVs.lookup(ExpandR->getSCEV()));
     ExpandR->replaceAllUsesWith(ExpandedVal);
     if (Plan.getTripCount() == ExpandR)
       Plan.resetTripCount(ExpandedVal);