[memprof] Initialize AllocInfoIter and CallSitesIter (NFC) (#124972)

This patch initializes AllocInfoIter and CallSitesIter to their
respective end().  I'm doing this not because I'm worried about
uninitialized iterators, but because the resulting code looks shorter
and makes it clear which data structure each iterator is associated
with.

Author: kazutakahirata

llvm/lib/Transforms/Instrumentation/MemProfiler.cpp

@@ -1096,9 +1096,8 @@ readMemprof(Module &M, Function &F, IndexedInstrProfReader *MemProfReader,
       // can happen because we don't currently have discriminators to
       // distinguish the case when a single line/col maps to both an allocation
       // and another callsite).
-      std::map<uint64_t, std::set<const AllocationInfo *>>::iterator
-          AllocInfoIter;
-      decltype(LocHashToCallSites)::iterator CallSitesIter;
+      auto AllocInfoIter = LocHashToAllocInfo.end();
+      auto CallSitesIter = LocHashToCallSites.end();
       for (const DILocation *DIL = I.getDebugLoc(); DIL != nullptr;
            DIL = DIL->getInlinedAt()) {
         // Use C++ linkage name if possible. Need to compile with