rtl-optimization/114855 - slow add_store_equivs in IRA

For the testcase in PR114855 at -O1 add_store_equivs shows up as the
main sink for bitmap_set_bit because it uses a bitmap to mark all
seen insns by UID to make sure the forward walk in memref_used_between_p
will find the insn in question.  Given we do have a CFG here the
functions operation is questionable, given memref_used_between_p
together with the walk of all insns is obviously quadratic in the
worst case that whole thing should be re-done ... but, for the
testcase, using a sbitmap of size get_max_uid () + 1 gets
bitmap_set_bit off the profile and improves IRA time from 15.58s (8%)
to 3.46s (2%).

Now, given above quadraticness I wonder whether we should instead
gate add_store_equivs on optimize > 1 or flag_expensive_optimizations.

	PR rtl-optimization/114855
	* ira.cc (add_store_equivs): Use sbitmap for tracking
	visited insns.

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