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.h

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