[Analysis] Avoid repeated hash lookups (NFC) (#127024)

Author: kazutakahirata

llvm/lib/Analysis/PhiValues.cpp

@@ -67,8 +67,10 @@ void PhiValues::processPhi(const PHINode *Phi,
       }
       // If the phi did not become part of a component then this phi and that
       // phi are part of the same component, so adjust the depth number.
-      if (!ReachableMap.count(OpDepthNumber))
-        DepthMap[Phi] = std::min(DepthMap[Phi], OpDepthNumber);
+      if (!ReachableMap.count(OpDepthNumber)) {
+        unsigned &Depth = DepthMap[Phi];
+        Depth = std::min(Depth, OpDepthNumber);
+      }
     } else {
       TrackedValues.insert(PhiValuesCallbackVH(PhiOp, this));
     }