More PRE TLC

This makes get_expr_value_id cheap and completes the
constant value-id simplification by turning the constant_value_expressions
into a direct map instead of a set of pre_exprs for the value.

2020-11-10  Richard Biener  <rguenther@suse.de>

	* tree-ssa-pre.c (pre_expr_d::value_id): Add.
	(constant_value_expressions): Turn into an array of pre_expr.
	(get_or_alloc_expr_for_nary): New function.
	(get_or_alloc_expr_for_reference): Likewise.
	(add_to_value): For constant values only ever add a single
	CONSTANT.
	(get_expr_value_id): Return the new value_id member.
	(vn_valnum_from_value_id): Split out and simplify constant
	value id handling.
	(get_or_alloc_expr_for_constant): Set the value_id member.
	(phi_translate_1): Use get_or_alloc_expr_for_*.
	(compute_avail): Likewise.
	(bitmap_find_leader): Simplify constant value id handling.