RFR(S): 8244721: CTW: C2 (Shenandoah) compilation fails with "unexpected infinite loop graph shape"

[Roland Westrelin]

Thanks for looking at this.

> Is there any reason why assert on L2141 does not subsume assert at L2139?
>
> 2138                   }
> 2139                   assert(mem != NULL, "should have found safepoint");
> 2140                 }
> 2141                 assert(mem != NULL, "should have found safepoint");
> 2142               } else {
> 2143                 mem = phi_mem;
> 2144               }

assert L2139 checks that every backedge has a safepoint.
assert L2141 checks that there was indeed a backedge.

Roland.