[memprof] Avoid repeated hash lookups (NFC) (#136268)

Note that we don't have to worry about CallstackProfileData[Id]
default-constructing the value side of a new map entry.  If that
happens, AccessHistogramSize > 0 wouldn't be true, and the new map
entry gets deleted right away.

Author: kazutakahirata

llvm/lib/ProfileData/MemProfReader.cpp

@@ -600,9 +600,12 @@ Error RawMemProfReader::symbolizeAndFilterStackFrames(
   // Drop the entries where the callstack is empty.
   for (const uint64_t Id : EntriesToErase) {
     StackMap.erase(Id);
-    if (CallstackProfileData[Id].AccessHistogramSize > 0)
-      free((void *)CallstackProfileData[Id].AccessHistogram);
-    CallstackProfileData.erase(Id);
+    if (auto It = CallstackProfileData.find(Id);
+        It != CallstackProfileData.end()) {
+      if (It->second.AccessHistogramSize > 0)
+        free((void *)It->second.AccessHistogram);
+      CallstackProfileData.erase(It);
+    }
   }
 
   if (StackMap.empty())