8275330: C2: assert(n->is_Root() || n->is_Region() || n->is_Phi() ||...

8275330: C2:  assert(n->is_Root() || n->is_Region() || n->is_Phi() || n->is_MachMerge() || def_block->dominates(block)) failed: uses must be dominated by definitions

Reviewed-by: thartmann, chagedorn