LibX86: Tolerate invalid segment register indices when disassembling

While #6 and #7 are not valid segment register, they can still be
encoded in otherwise-valid instructions, so let's tolerate it.

Libraries/LibX86/Instruction.cpp

@@ -1760,6 +1760,10 @@ const char* register_name(SegmentRegister index)
         return "fs";
     case SegmentRegister::GS:
         return "gs";
+    case SegmentRegister::SegR6:
+        return "segr6";
+    case SegmentRegister::SegR7:
+        return "segr7";
     default:
         ASSERT_NOT_REACHED();
         return nullptr;

Libraries/LibX86/Instruction.h

@@ -88,6 +88,8 @@ enum class SegmentRegister {
     DS,
     FS,
     GS,
+    SegR6,
+    SegR7,
     None = 0xFF,
 };