make PRE constant value IDs negative

This separates constant and non-constant value-ids to allow for
a more efficient constant_value_id_p and for more efficient bit-packing
inside the bitmap sets which never contain any constant values.

There's further optimization opportunities but at this stage
I'll do small refactorings.

2020-11-06  Richard Biener  <rguenther@suse.de>

	* tree-ssa-sccvn.h (get_max_constant_value_id): Declare.
	(get_next_constant_value_id): Likewise.
	(value_id_constant_p): Inline and simplify.
	* tree-ssa-sccvn.c (constant_value_ids): Remove.
	(next_constant_value_id): Add.
	(get_or_alloc_constant_value_id): Adjust.
	(value_id_constant_p): Remove definition.
	(get_max_constant_value_id): Define.
	(get_next_value_id): Add assert for overflow.
	(get_next_constant_value_id): Define.
	(run_rpo_vn): Adjust.
	(free_rpo_vn): Likewise.
	(do_rpo_vn): Initialize next_constant_value_id.
	* tree-ssa-pre.c (constant_value_expressions): New.
	(add_to_value): Split into constant/non-constant value
	handling.  Avoid exact re-allocation.
	(vn_valnum_from_value_id): Adjust.
	(phi_translate_1): Remove spurious exact re-allocation.
	(bitmap_find_leader): Adjust.  Make sure we return
	a CONSTANT value for a constant value id.
	(do_pre_regular_insertion): Use 2 auto-elements for avail.
	(do_pre_partial_partial_insertion): Likewise.
	(init_pre): Allocate constant_value_expressions.
	(fini_pre): Release constant_value_expressions.

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