[OpenMPOpt] deduplicateRuntimeCalls(): avoid traditional map lookup pitfall

[lebedev.ri]

Summary

This roughly halves time spent in that pass,
while unsurprisingly significantly reducing total memory usage.

This makes sense because most functions won't use any openmp functions..

old

0.2329 (  0.5%)   0.0409 (  0.9%)   0.2738 (  0.5%)   0.2736 (  0.5%)  OpenMP specific optimizations

total runtime: 63.32s.
bytes allocated in total (ignoring deallocations): 8.34GB (131.70MB/s)
calls to allocation functions: 14526259 (229410/s)
temporary memory allocations: 3335760 (52680/s)
peak heap memory consumption: 324.36MB
peak RSS (including heaptrack overhead): 5.39GB
total memory leaked: 289.93MB

new

0.1457 (  0.3%)   0.0276 (  0.6%)   0.1732 (  0.3%)   0.1731 (  0.3%)  OpenMP specific optimizations

total runtime: 55.01s.
bytes allocated in total (ignoring deallocations): 6.70GB (121.89MB/s)
calls to allocation functions: 14268205 (259398/s)
temporary memory allocations: 3225355 (58637/s)
peak heap memory consumption: 324.09MB
peak RSS (including heaptrack overhead): 5.39GB
total memory leaked: 289.87MB

diff

total runtime: -8.31s.
bytes allocated in total (ignoring deallocations): -1.63GB (196.58MB/s)
calls to allocation functions: -258054 (31034/s)
temporary memory allocations: -110405 (13277/s)
peak heap memory consumption: -262.36KB
peak RSS (including heaptrack overhead): 0B
total memory leaked: -61.45KB

[jdoerfert]

LGTM. Thanks!

llvm/lib/Transforms/IPO/OpenMPOpt.cpp

Value *getCombinedIdentFromCallUsesIn(RuntimeFunctionInfo &RFI, Function &F,
     }
     return Ident;
   }
 
   /// Try to eliminiate calls of \p RFI in \p F by reusing an existing one or
   /// \p ReplVal if given.
   bool deduplicateRuntimeCalls(Function &F, RuntimeFunctionInfo &RFI,
                                Value *ReplVal = nullptr) {
-    auto &Uses = RFI.UsesMap[&F];
+    auto UsesIt = RFI.UsesMap.find(&F);
+    if (UsesIt == RFI.UsesMap.end())
+      return false;
+
+    auto &Uses = UsesIt->getSecond();
     if (Uses.size() + (ReplVal != nullptr) < 2)
       return false;
 
     LLVM_DEBUG(dbgs() << TAG << "Deduplicate " << Uses.size() << " uses of "
                       << RFI.Name
                       << (ReplVal ? " with an existing value\n" : "\n")
                       << "\n");
     assert((!ReplVal || (isa<Argument>(ReplVal) &&