middle-end: support SLP early break

This patch introduces feature parity for early break int the SLP only
vectorizer.

The approach taken here is to treat the early exits as root statements for an
SLP tree.  This means that we don't need any changes to build_slp to support
gconds.

Codegen for the gcond itself now has to be done out of line but the body of the
SLP blocks itself is simply driven by SLP scheduling.  There is a slight
awkwardness in having re-used vectorizable_early_exit for both SLP and non-SLP
but I've documented the differences and when I did try to refactor it it wasn't
really worth it given that this is a temporary state anyway.

This version is restricted to lane = 1, as such we can re-use the existing
move_early_break function instead of having to do safety update through
scheduling.  I have a branch where I'm working on that but lane > 1 is out of
scope for GCC 15 anyway.   The only reason I will try to get moving through
scheduling done as a stretch goal is so we get epilogue vectorization back for
early break.

The example:

unsigned test4(unsigned x)
{
 unsigned ret = 0;
 for (int i = 0; i < N; i++)
 {
   vect_b[i] = x + i;
   if (vect_a[i]*2 != x)
     break;
   vect_a[i] = x;

 }
 return ret;
}

builds the following SLP instance for early break:

note:   Analyzing vectorizable control flow: if (patt_6 != 0)
note:   Starting SLP discovery for
note:     patt_6 = _4 != x_9(D);
note:   starting SLP discovery for node 0x63abc80
note:   Build SLP for patt_6 = _4 != x_9(D);
note:   precomputed vectype: vector(4) <signed-boolean:32>
note:   nunits = 4
note:   vect_is_simple_use: operand x_9(D), type of def: external
note:   vect_is_simple_use: operand # RANGE [irange] unsigned int [0, 0][2, +INF] MASK 0xffff
        _3 * 2, type of def: internal
note:   starting SLP discovery for node 0x63abdc0
note:   Build SLP for _4 = _3 * 2;
note:   precomputed vectype: vector(4) unsigned int
note:   nunits = 4
note:   vect_is_simple_use: operand #
        vect_aD.4416[i_15], type of def: internal
note:   vect_is_simple_use: operand 2, type of def: constant
note:   starting SLP discovery for node 0x63abe60
note:   Build SLP for _3 = vect_a[i_15];
note:   precomputed vectype: vector(4) unsigned int
note:   nunits = 4
note:   SLP discovery for node 0x63abe60 succeeded
note:   SLP discovery for node 0x63abdc0 succeeded
note:   SLP discovery for node 0x63abc80 succeeded
note:   SLP size 3 vs. limit 10.
note:   Final SLP tree for instance 0x6474190:
note:   node 0x63abc80 (max_nunits=4, refcnt=2) vector(4) <signed-boolean:32>
note:   op template: patt_6 = _4 != x_9(D);
note:   	stmt 0 patt_6 = _4 != x_9(D);
note:   	children 0x63abd20 0x63abdc0
note:   node (external) 0x63abd20 (max_nunits=1, refcnt=1)
note:   	{ x_9(D) }
note:   node 0x63abdc0 (max_nunits=4, refcnt=2) vector(4) unsigned int
note:   op template: _4 = _3 * 2;
note:   	stmt 0 _4 = _3 * 2;
note:   	children 0x63abe60 0x63abf00
note:   node 0x63abe60 (max_nunits=4, refcnt=2) vector(4) unsigned int
note:   op template: _3 = vect_a[i_15];
note:   	stmt 0 _3 = vect_a[i_15];
note:   	load permutation { 0 }
note:   node (constant) 0x63abf00 (max_nunits=1, refcnt=1)
note:   	{ 2 }

and during codegen:

note:   ------>vectorizing SLP node starting from: patt_6 = _4 != x_9(D);
note:   vect_is_simple_use: operand # RANGE [irange] unsigned int [0, 0][2, +INF] MASK 0xffff
        _3 * 2, type of def: internal
note:   add new stmt: mask_patt_6.18_58 = _53 != vect__4.17_57;
note:    === vectorizable_early_exit ===
note:    transform early-exit.
note:   vectorizing stmts using SLP.
note:   Vectorizing SLP tree:
note:   node 0x63abfa0 (max_nunits=4, refcnt=1) vector(4) int
note:   op template: i_12 = i_15 + 1;
note:   	stmt 0 i_12 = i_15 + 1;
note:   	children 0x63aba00 0x63ac040
note:   node 0x63aba00 (max_nunits=4, refcnt=2) vector(4) int
note:   op template: i_15 = PHI <i_12(6), 0(14)>
note:   	[l] stmt 0 i_15 = PHI <i_12(6), 0(14)>
note:   	children (nil) (nil)
note:   node (constant) 0x63ac040 (max_nunits=1, refcnt=1) vector(4) int
note:   	{ 1 }

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_analyze_loop_2): Handle SLP trees with no
	children.
	* tree-vectorizer.h (enum slp_instance_kind): Add slp_inst_kind_gcond.
	(LOOP_VINFO_EARLY_BREAKS_LIVE_IVS): New.
	(vectorizable_early_exit): Expose.
	(class _loop_vec_info): Add early_break_live_stmts.
	* tree-vect-slp.cc (vect_build_slp_instance, vect_analyze_slp_instance):
	Support gcond instances.
	(vect_analyze_slp): Analyze gcond roots and early break live statements.
	(maybe_push_to_hybrid_worklist): Don't sink gconds.
	(vect_slp_analyze_operations): Support gconds.
	(vect_slp_check_for_roots): Update comments.
	(vectorize_slp_instance_root_stmt): Support gconds.
	(vect_schedule_slp): Pass vinfo to vectorize_slp_instance_root_stmt.
	* tree-vect-stmts.cc (vect_stmt_relevant_p): Record early break live
	statements.
	(vectorizable_early_exit): Support SLP.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-early-break_126.c: New test.
	* gcc.dg/vect/vect-early-break_127.c: New test.
	* gcc.dg/vect/vect-early-break_128.c: New test.