reorg.c (fill_slots_from_thread): Improve for TARGET_FLAGS_REGNUM

This handles TARGET_FLAGS_REGNUM clobbering insns as delay-slot
fillers using a method similar to that in commit 33c2207d3fda,
where care was taken for fill_simple_delay_slots to allow such
insns when scanning for delay-slot fillers *backwards* (before
the insn).

A TARGET_FLAGS_REGNUM target is typically a former cc0 target.
For cc0 targets, insns don't mention clobbering cc0, so the
clobbers are mentioned in the "resources" only as a special
entity and only for compare-insns and branches, where the cc0
value matters.

In contrast, with TARGET_FLAGS_REGNUM, most insns clobber it and
the register liveness detection in reorg.c / resource.c treats
that as a blocker (for other insns mentioning it, i.e. most)
when looking for delay-slot-filling candidates.  This means that
when comparing core and performance for a delay-slot cc0 target
before and after the de-cc0 conversion, the inability to fill a
delay slot after conversion manifests as a regression.  This was
one such case, for CRIS, with random_bitstring in
gcc.c-torture/execute/arith-rand-ll.c as well as the target
libgcc division function.

After this, all known performance regressions compared to cc0
are fixed.

gcc:
	PR target/93372
	* reorg.c (fill_slots_from_thread): Allow trial insns that clobber
	TARGET_FLAGS_REGNUM as delay-slot fillers.

gcc/testsuite:
	PR target/93372
	* gcc.target/cris/pr93372-47.c: New test.

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