[llvm-exegesis][X86] Ignore a few unmeasurable opcodes.

llvm/tools/llvm-exegesis/lib/X86/Target.cpp

 // the reason. nullptr indicates a valid opcode.
 static const char *isInvalidOpcode(const Instruction &Instr) {
   const auto OpcodeName = Instr.Name;
   if ((Instr.Description.TSFlags & X86II::FormMask) == X86II::Pseudo)
     return "unsupported opcode: pseudo instruction";
   if (OpcodeName.startswith("POP") || OpcodeName.startswith("PUSH") ||
       OpcodeName.startswith("ADJCALLSTACK") || OpcodeName.startswith("LEAVE"))
     return "unsupported opcode: Push/Pop/AdjCallStack/Leave";
+  switch (Instr.Description.Opcode) {
+  case X86::LFS16rm:
+  case X86::LFS32rm:
+  case X86::LFS64rm:
+  case X86::LGS16rm:
+  case X86::LGS32rm:
+  case X86::LGS64rm:
+  case X86::LSS16rm:
+  case X86::LSS32rm:
+  case X86::LSS64rm:
+  case X86::SYSENTER:
+    return "unsupported opcode";
+  default:
+    break;
+  }
   if (const auto reason = isInvalidMemoryInstr(Instr))
     return reason;
   // We do not handle instructions with OPERAND_PCREL.
   for (const Operand &Op : Instr.Operands)
     if (Op.isExplicit() &&
         Op.getExplicitOperandInfo().OperandType == MCOI::OPERAND_PCREL)
       return "unsupported opcode: PC relative operand";
   // We do not handle second-form X87 instructions. We only handle first-form