RFR: Fix ExtendedDTraceProbes with Shenandoah

[Roman Kennke]

> 
>> The interesting question is: is this reproducible with upstream+G1
>> somehow? Can you make a testcase against G1, and fix it upstream?
> 
> It's usually quite hard to write a simple test case for a very specific
> reg alloc problem. I will submit it upstream anyway.

We have one that fails for us reliably, right? We can turn that into G1
testcase, I suppose? Zhengyu?


>> Also, might we have more 'hidden-ctrl-flow-bugs' lurking in our code?
> 
> It's quite possible. Hidden ctrl flow constructs are quite error prone
> and fragile.

Yeah, we should check at least our own stuff.

Roman