[llvm] [MCA] Avoid repeated hash lookups (NFC) (PR #129192)

[Kazu Hirata via llvm-commits]

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@@ -634,16 +634,14 @@ InstrBuilder::createInstrDescImpl(const MCInst &MCI,
   bool IsVariadic = MCDesc.isVariadic();
   if ((ID->IsRecyclable = !IsVariadic && !IsVariant)) {
     auto DKey = std::make_pair(MCI.getOpcode(), SchedClassID);
-    Descriptors[DKey] = std::move(ID);
-    return *Descriptors[DKey];
+    return *(Descriptors[DKey] = std::move(ID));
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kazutakahirata wrote:

I could use `insert_or_assign`, but that would be a bit mouthful though:

```
    return *(Descriptors[DKey] = std::move(ID));
    return *(Descriptors.insert_or_assign(DKey, std::move(ID)).first->second);
```


https://github.com/llvm/llvm-project/pull/129192