[10] RFR(M): 8176506: C2: loop unswitching and unsafe accesses cause crash

[Roland Westrelin]

> Here is a new webrev. Halt now should trigger a SIGILL on all
> platforms. I tested x86, aarch64, arm64. The code for arm32 needs to be
> tested (I can't even build on arm32).
>
> http://cr.openjdk.java.net/~roland/8176506/webrev.04/

Tobias verified the change on arm32 (thanks Tobias!). So AFAICT, it's
good to go. Anyone to sponsor it?

Roland.