tree-optimization: Clarify LIM memory ref equality

tree-optimization: Clarify LIM memory ref equality

Restructure mem_ref_hasher::equal so the non-decomposed fallback
and the decomposed comparison path are explicit. The non-decomposed
lookup still compares the original refs with operand_equal_p, while
the decomposed path keeps the same base, offset, size, max_size,
volatile, alias-set, and type checks as before. This matches the split
in gather_mem_refs_stmt, where non-decomposed references are marked
with an unknown max_size and decomposed references are hashed from
base, offset, and size.

	* tree-ssa-loop-im.cc (mem_ref_hasher::equal): Refactor
	for clarity.

Signed-off-by: Xin Wang <wangxinw@hygon.cn>