vect: Respect slp decision when applying suggested uf [PR105940]

vect: Respect slp decision when applying suggested uf [PR105940]

This follows Richi's suggestion in PR105940, it aims to avoid
inconsistent slp decision between when the suggested unroll
factor is worked out and when the suggested unroll factor is
applied.

If the previous slp decision is true when the suggested unroll
factor is worked out, when we are applying unroll factor we
don't need to start over with slp off if the analysis with slp
on fails.  On the other hand, if the previous slp decision is
false when the suggested unroll factor is worked out, when we
are applying unroll factor we can skip the slp handlings.

Function vect_is_simple_reduction saves reduction chains for
subsequent slp analyses, we have to disable this early otherwise
there is an ICE in vectorizable_reduction for below:

  if (REDUC_GROUP_FIRST_ELEMENT (stmt_info))
    gcc_assert (slp_node
		&& REDUC_GROUP_FIRST_ELEMENT (stmt_info)
		   == stmt_info);

	PR tree-optimization/105940

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_analyze_loop_2): Add new parameter
	slp_done_for_suggested_uf and adjust with it accordingly.
	(vect_analyze_loop_1): Add new variable slp_done_for_suggested_uf,
	pass it down to vect_analyze_loop_2 for the initial analysis and
	applying suggested unroll factor.
	(vect_is_simple_reduction): Add parameter slp and adjust with it.
	(vect_analyze_scalar_cycles_1): Add parameter slp and pass down.
	(vect_analyze_scalar_cycles): Likewise.