tree-optimization/101555 - avoid redundant alias queries in PRE

tree-optimization/101555 - avoid redundant alias queries in PRE

This avoids doing redundant work during PHI translation to invalidate
mems when translating their corresponding VUSE through the blocks
virtual PHI node.  All the invalidation work is already done by
prune_clobbered_mems.

This speeds up the compile of the testcase from 275s with PRE
taking 91% of the compile-time down to 43s with PRE taking 16%
of the compile-time.

2021-09-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101555
	* tree-ssa-pre.c (translate_vuse_through_block): Do not
	perform an alias walk to determine the validity of the
	mem at the start of the block which is already guaranteed
	by means of prune_clobbered_mems.
	(phi_translate_1): Pass edge to translate_vuse_through_block.