[memprof] Use const ref for IndexedRecord (#94114)

The type of *Iter here is "const IndexedMemProfRecord &" as defined in
RecordLookupTrait.  Assigning *Iter to a variable of type
"const IndexedMemProfRecord &" avoids a copy, reducing the cycle and
instruction counts by 1.8% and 0.2%, respectively, with
"llvm-profdata show" modified to deserialize all MemProfRecords.

Note that RecordLookupTrait has an internal copy of
IndexedMemProfRecord, so we don't have to worry about a dangling
reference to a temporary.

Author: kazutakahirata

llvm/lib/ProfileData/InstrProfReader.cpp

@@ -1613,7 +1613,7 @@ IndexedMemProfReader::getMemProfRecord(const uint64_t FuncNameHash) const {
         instrprof_error::unknown_function,
         "memprof record not found for function hash " + Twine(FuncNameHash));
 
-  const memprof::IndexedMemProfRecord IndexedRecord = *Iter;
+  const memprof::IndexedMemProfRecord &IndexedRecord = *Iter;
   switch (Version) {
   case memprof::Version0:
   case memprof::Version1: