tree-optimization/101293 - further enhance LIMs ref canonicalization

tree-optimization/101293 - further enhance LIMs ref canonicalization

This makes sure to handle MEM[p + 4] and MEM[p].j with j at offset 4
as the same ref in store motion.  For hashing we need to be
more restrictive in what we handle since there's no poly-int
handlers for inchash.  For comparison we can compare poly_offsets directly.

2021-07-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101293
	* tree-ssa-loop-im.c (mem_ref_hasher::equal): Compare MEM_REF bases
	with combined offsets.
	(gather_mem_refs_stmt): Hash MEM_REFs as if their offset were
	combined with the rest of the offset.

	* gcc.dg/tree-ssa/ssa-lim-15.c: New testcase.