rtl-ssa: Fix prev/next_def confusion [PR104869]

rtl-ssa chains definitions into an RPO list.  It also groups
sequences of clobbers together into a single node, so that it's
possible to skip over the clobbers in constant time in order to
get the next or previous set.

When adding a clobber to an insn, the main DF barriers for that
clobber are the last use of the previous set (if any) and the next
set (if any); adding a new clobber to a sea of clobbers is fine.
def_lookup provided the basis for these barriers as prev_def ()
and next_def ().

But of course, in hindsight, those were bad names, since they
implied that the returned values were literally the previous
definition (of any kind) or the next definition (of any kind).
And function_info::make_use_available was using the same routines
assuming that they had that meaning. :-(

This made a difference for the case where the start of a BB
occurs in the middle of an (RPO) clobber group: we then want
the previous and next clobbers in the group, rather than the
set before the clobber group and the set after the clobber group.

This patch renames the existing routines to something that's hopefully
clearer (though also more long-winded).  It then adds routines that
really do provide the previous and next definitions.

This complication is supposed to be internal to rtl-ssa and, as
mentioned above, is part of trying to reduce time complexity.

gcc/
	PR middle-end/104869
	* rtl-ssa/accesses.h (clobber_group::prev_clobber): Declare.
	(clobber_group::next_clobber): Likewise.
	(def_lookup::prev_def): Rename to...
	(def_lookup::last_def_of_prev_group): ...this.
	(def_lookup::next_def): Rename to...
	(def_lookup::first_def_of_next_group): ...this.
	(def_lookup::matching_or_prev_def): Rename to...
	(def_lookup::matching_set_or_last_def_of_prev_group): ...this.
	(def_lookup::matching_or_next_def): Rename to...
	(def_lookup::matching_set_or_first_def_of_next_group): ...this.
	(def_lookup::prev_def): New function, taking the lookup insn as
	argument.
	(def_lookup::next_def): Likewise.
	* rtl-ssa/member-fns.inl (def_lookup::prev_def): Rename to...
	(def_lookup::last_def_of_prev_group): ...this.
	(def_lookup::next_def): Rename to...
	(def_lookup::first_def_of_next_group): ...this.
	(def_lookup::matching_or_prev_def): Rename to...
	(def_lookup::matching_set_or_last_def_of_prev_group): ...this.
	(def_lookup::matching_or_next_def): Rename to...
	(def_lookup::matching_set_or_first_def_of_next_group): ...this.
	* rtl-ssa/movement.h (restrict_movement_for_dead_range): Update after
	above renaming.
	* rtl-ssa/accesses.cc (clobber_group::prev_clobber): New function.
	(clobber_group::next_clobber): Likewise.
	(def_lookup::prev_def): Likewise.
	(def_lookup::next_def): Likewise.
	(function_info::make_use_available): Pass the lookup insn to
	def_lookup::prev_def and def_lookup::next_def.

gcc/testsuite/
	PR middle-end/104869
	* g++.dg/pr104869.C: New test.