rework PRE PHI translation cache

Turns out its size and time requirements can be stripped down
dramatically.

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

	* tree-ssa-pre.c (expr_pred_trans_d): Modify so elements
	are embedded rather than allocated.  Remove hashval member,
	make all members integers.
	(phi_trans_add): Adjust accordingly.
	(phi_translate): Likewise.  Deal with re-allocation
	of the table.