[LV] Simplify register usage code and avoid double map lookups.

[craig.topper]

Summary

Instead of checking whether a map entry exists to decide if we should
initialize it or add to it, we can rely on the map entry being constructed
and initialized to 0 before the addition happens.

For the std::max case, I've made a reference to the map entry to
avoid looking it up twice.

[reames]

LGTM

Please tag title as nfc.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

for (unsigned int i = 0, s = IdxToInstr.size(); i < s; ++i) {
     // For each VF find the maximum usage of registers.
     for (unsigned j = 0, e = VFs.size(); j < e; ++j) {
       // Count the number of live intervals.
       SmallMapVector<unsigned, unsigned, 4> RegUsage;
 
       if (VFs[j].isScalar()) {
         for (auto *Inst : OpenIntervals) {
           unsigned ClassID = TTI.getRegisterClassForType(false, Inst->getType());
-          if (RegUsage.find(ClassID) == RegUsage.end())
-            RegUsage[ClassID] = 1;
-          else
-            RegUsage[ClassID] += 1;
+          // If RegUsage[ClassID] doesn't exist, it will be default
+          // constructed as 0 before the addition
         }
       } else {
         collectUniformsAndScalars(VFs[j]);
         for (auto *Inst : OpenIntervals) {
           // Skip ignored values for VF > 1.
           if (VecValuesToIgnore.count(Inst))
             continue;
           if (isScalarAfterVectorization(Inst, VFs[j])) {
             unsigned ClassID = TTI.getRegisterClassForType(false, Inst->getType());
-            if (RegUsage.find(ClassID) == RegUsage.end())
-              RegUsage[ClassID] = 1;
-            else
-              RegUsage[ClassID] += 1;
+            // If RegUsage[ClassID] doesn't exist, it will be default
+            // constructed as 0 before the addition
           } else {
             unsigned ClassID = TTI.getRegisterClassForType(true, Inst->getType());
-            if (RegUsage.find(ClassID) == RegUsage.end())
-              RegUsage[ClassID] = GetRegUsage(Inst->getType(), VFs[j]);
-            else
-              RegUsage[ClassID] += GetRegUsage(Inst->getType(), VFs[j]);
+            // If RegUsage[ClassID] doesn't exist, it will be default
+            // constructed as 0 before the addition
           }
         }
       }
 
       for (auto& pair : RegUsage) {
-        if (MaxUsages[j].find(pair.first) != MaxUsages[j].end())
-          MaxUsages[j][pair.first] = std::max(MaxUsages[j][pair.first], pair.second);
-        else
-          MaxUsages[j][pair.first] = pair.second;
+        auto &Entry = MaxUsages[j][pair.first];
+        Entry = std::max(Entry, pair.second);
       }
     }
 
     LLVM_DEBUG(dbgs() << "LV(REG): At #" << i << " Interval # "
                       << OpenIntervals.size() << '\n');
 
     // Add the current instruction to the list of open intervals.
     OpenIntervals.insert(I);