[llvm-exegesis] Restrict to allowed back-to-back instructions in SerialSnippetGenerator.

[courbet]

Summary

Followup to D73161.

llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp

 //===-- SerialSnippetGenerator.cpp ------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "SerialSnippetGenerator.h"
 
-#include "MCInstrDescView.h"
 #include "CodeTemplate.h"
+#include "MCInstrDescView.h"
+#include "Target.h"
 #include <algorithm>
 #include <numeric>
 #include <vector>
 
 namespace llvm {
 namespace exegesis {
 
 struct ExecutionClass {
computeAliasingInstructions(const LLVMState &State, const Instruction *Instr,
 
   std::vector<const Instruction *> AliasingInstructions;
   for (const unsigned OtherOpcode : Opcodes) {
     if (OtherOpcode == Instr->Description.getOpcode())
       continue;
     const Instruction &OtherInstr = State.getIC().getInstr(OtherOpcode);
     if (OtherInstr.hasMemoryOperands())
       continue;
+    if (!State.getExegesisTarget().allowAsBackToBack(OtherInstr))
+      continue;
     if (Instr->hasAliasingRegistersThrough(OtherInstr, ForbiddenRegisters))
       AliasingInstructions.push_back(&OtherInstr);
     if (AliasingInstructions.size() >= MaxAliasingInstructions)
       break;
   }
   return AliasingInstructions;
 }