tree-optimization/93508 - make VN translate through _chk and valueize length

tree-optimization/93508 - make VN translate through _chk and valueize length

Value-numbering failed to handle __builtin_{memcpy,memset,...}_chk
variants when removing abstraction and also failed to use the
value-numbering lattice when requiring the length argument of the
call to be constant.

2020-02-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93508
	* tree-ssa-sccvn.c (vn_reference_lookup_3): Handle _CHK like
	non-_CHK variants.  Valueize their length arguments.

	* gcc.dg/tree-ssa/ssa-fre-85.c: New testcase.