stabilize store merging

stabilize store merging

Don't let pointer randomization change the order in which we process
store chains.  This may cause SSA_NAMEs to be released in different
order, and if they're reused later, they may cause differences in SSA
partitioning, leading to differences in expand, and ultimately to
different code.

bootstrap-debug-lean (-fcompare-debug) on i686-linux-gnu has failed in
haifa-sched.c since r245196 exposed the latent ordering problem in
store merging.  In this case, the IR differences (different SSA names
selected for copies in out-of-SSA, resulting in some off-by-one
differences in pseudos) were not significant enough to be visible in
the compiler output.

for  gcc/ChangeLog

	* gimple-ssa-store-merging.c (struct imm_store_chain_info):
	Add linked-list forward and backlinks.  Insert on
	construction, remove on destruction.
	(class pass_store_merging): Add m_stores_head field.
	(pass_store_merging::terminate_and_process_all_chains):
	Iterate over m_stores_head list.
	(pass_store_merging::terminate_all_aliasing_chains):
	Likewise.
	(pass_store_merging::execute): Check for debug stmts first.
	Push new chains onto the m_stores_head stack.

From-SVN: r246213