RFR: Fix ExtendedDTraceProbes with Shenandoah

[Roland Westrelin]

> 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.

> 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.

Roland.