Improve bytewise DSE

Improve bytewise DSE

testcase modref-dse-4.c and modref-dse-5.c fails on some targets because they
depend on store merging.  What really happens is that without store merging
we produce for kill_me combined write that is ao_ref with offset=0, size=32
and max_size=96.  We have size != max_size becaue we do ont track the info that
all 3 writes must happen in a group and conider case only some of them are done.

This disables byte-wise DSE which checks that size == max_size.  This is
completely unnecesary for store being proved to be dead or load being checked
to not read live bytes.  It is only necessary for kill store that is used to
prove that given store is dead.

While looking into this I also noticed that we check that everything is byte
aligned.  This is also unnecessary and with access merging in modref may more
commonly fire on accesses that we could otherwise handle.

This patch fixes both also also changes interface to normalize_ref that I found
confusing since it modifies the ref. Instead of that we have get_byte_range
that is computing range in bytes (since that is what we need to maintain the
bitmap) and has additional parameter specifying if the store in question should
be turned into sub-range or super-range depending whether we compute range
for kill or load.

gcc/ChangeLog:

2021-11-23  Jan Hubicka  <hubicka@ucw.cz>

	PR tree-optimization/103335
	* tree-ssa-dse.c (valid_ao_ref_for_dse): Rename to ...
	(valid_ao_ref_kill_for_dse): ... this; do not check that boundaries
	are divisible by BITS_PER_UNIT.
	(get_byte_aligned_range_containing_ref): New function.
	(get_byte_aligned_range_contained_in_ref): New function.
	(normalize_ref): Rename to ...
	(get_byte_range): ... this one; handle accesses not aligned to byte
	boundary; return range in bytes rater than updating ao_ref.
	(clear_live_bytes_for_ref): Take write ref by reference; simplify using
	get_byte_access.
	(setup_live_bytes_from_ref): Likewise.
	(clear_bytes_written_by): Update.
	(live_bytes_read): Update.
	(dse_classify_store): Simplify tech before live_bytes_read checks.

gcc/testsuite/ChangeLog:

2021-11-23  Jan Hubicka  <hubicka@ucw.cz>

	* gcc.dg/tree-ssa/modref-dse-4.c: Update template.
	* gcc.dg/tree-ssa/modref-dse-5.c: Update template.