* passes.def: Add arg to pass_dominator pass instantiation.
* tree-pass.h (first_pass_instance): Remove pass_dominator-related bit
of comment.
* tree-ssa-dom.c (pass_dominator::pass_dominator): Initialize
may_peel_loop_headers_p.
(pass_dominator::set_pass_param): New member function. Set
may_peel_loop_headers_p.
(pass_dominator::may_peel_loop_headers_p): New private member.
(pass_dominator::execute): Use may_peel_loop_headers_p instead of
first_pass_instance.