ASSERT: missing decode type in core/arch/x86/disassemble.c:227

In order to log the actual instructions observed at runtime, one can enable loglevel 3 by issuing the following command:

```
drrun -debug -loglevel 3 -c any_client -- any_app
```
This command runs without problems dumping the instructions in DR's own syntax. However, changing the disassembly syntax to `intel` or `att`, by adding command line option `-syntax_intel` or `-syntax_att` respectively, would trigger the above mentioned assertion which can be found [here](https://github.com/DynamoRIO/dynamorio/blob/master/core/arch/x86/disassemble.c#L228).

I've debugged this issue a bit and it seems like the assertion is triggered by instruction `syscall` which is called by `ld-linux-x86-64.so.2`. The instruction `syscall` does not have a matching `optype`. The issue is independent of the client used or the instrumented application since it's triggered by the loader.

I'm using DR 6.2.0 on an x86_64 machine running Ubuntu 16.04

Start in core/arch/x86/disassemble.c around line 228 and reproduce the assertion with drrun -debug -loglevel 3 using -syntax_intel or -syntax_att. Check how the syscall instruction is handled when it lacks a matching optype. Done means the loader no longer triggers the assertion and runtime instructions are logged in the requested syntax.

Written by the indexing model from the issue text.