tool-added jecxz;jmp_short;jmp messes up DR by triggering instr_is_cti_short_rewrite()

For #2820 I'm adding instrumentation that uses jecxz but can't always
reach, so I used the landing pad pattern jecxz;jmp_short;jmp, but that
fails to run and hits a DR assert:

```
$ clients/bin64/tool.drcacheoff.burst_threadfilter
pre-DR init
pre-DR start

```

It matches instr_is_cti_short_rewrite() and confuses DR.

For now I'm putting a nop after the jmp_short which won't be executed but
is a waste of space.