Fix DoGcStress for multireg return with GC pointers.

Fix DoGcStress for multireg return with GC pointers.

Note: we are overprotecting registers here, for example, we always protect 2 registers even if only one is returned. GCStress infrastructure should be able to tolerate extra registers. We have had such examples before (for example when returned a struct without pointers we protected the first reg anyway).