[SLPVectorizer] Avoid two successive hash lookups on the same key (#107143)

This patch replaces the find-try_emplace sequence with just one call
to try_emplace, thereby avoiding two successive hash lookups on the
same key.  I am not using the "inserted" boolean from try_emplace to
preserve the original behavior (that is, before PR 107123) that checks
to see if the value is nullptr or not.

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -11804,10 +11804,9 @@ void BoUpSLP::reorderInputsAccordingToOpcode(ArrayRef<Value *> VL,
 }
 
 Instruction &BoUpSLP::getLastInstructionInBundle(const TreeEntry *E) {
-  auto It = EntryToLastInstruction.find(E);
-  if (It != EntryToLastInstruction.end())
-    return *It->second;
   auto &Res = EntryToLastInstruction.try_emplace(E).first->second;
+  if (Res)
+    return *Res;
   // Get the basic block this bundle is in. All instructions in the bundle
   // should be in this block (except for extractelement-like instructions with
   // constant indeces).