MikePall

Below is the -jdump (there's no isolated testcase) that triggered this fix:

https://github.com/LuaJIT/LuaJIT/commit/2bc63bb6affdb378c1698bd0f97bacb286a61a6f

Reasoning for the interim fix:

Trace 555 has a code generation bug. It must not coalesce with r13 as BASE.
So IR(REF_BASE)->r of trace 555 contains r13. But the RETF in trace 444
modifies BASE with rdx right before exit 4 which leads to trace 555. Trace