[llvm] [SLPVectorizer] Use DenseMap::operator[] (NFC) (PR #107123)

[Kazu Hirata via llvm-commits]

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@@ -11841,9 +11841,9 @@ void BoUpSLP::reorderInputsAccordingToOpcode(ArrayRef<Value *> VL,
 }
 
 Instruction &BoUpSLP::getLastInstructionInBundle(const TreeEntry *E) {
-  auto &Res = EntryToLastInstruction.FindAndConstruct(E);
-  if (Res.second)
-    return *Res.second;
+  auto &Res = EntryToLastInstruction[E];
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kazutakahirata wrote:

`DenseMap::find` won't default-construct.  Would you be OK with something like this?

```
auto It = EntryToLastInstruction.find(E);
if (It != EntryToLastInstruction.end())
  return *It->second;
auto &Res = EntryToLastInstruction.insert(E).first->second;
```


https://github.com/llvm/llvm-project/pull/107123