[PERF2BOLT] Fix aggregator wrt traces with REP RET

Summary:
Previously the perf2bolt aggregator was rejecting traces
finishing with REP RET (return instruction with REP prefix) as a
result of the migration from objdump output to LLVM disassembler,
which decodes REP as a separate instruction. Add code to detect
REP RET and treat it as a single return instruction.

(cherry picked from FBD6417496)