forwprop: Fix lane handling for VEC_PERM sequence blending

In PR117830 a miscompilation of 464.h264ref was reported.
An analysis showed that wrong code was generated because of
unsatisfied assumptions.  This patch addresses these issues.

The first assumption was that we could independently analyze the two
vec-perms at the start of a vec-perm-simplify sequence and use the
information  later for calculating a final vec-perm selector that
utilizes fewer lanes.  However, this information does not help much,
because for changing the selector entry, we need to ensure that both
elements of the operand vectors v_1 and v_2 remain equal.
This is addressed by removing the function get_vect_selector_index_map
and checking for this equality in the loop where we create the new
selector.

The calculation of the selector vector for the blended sequence
assumed that the indices of the selector vector of the narrowed
sequences are increasing.  This assumption does not hold in general.
This was fixed by allowing a wrap-around when searching for an empty
lane.

Further, there was an issue in the calculation of the selector vector
entries for the second sequence.  The code did not consider that the
lanes of the second sequence could have been moved.

A relevant property of this patch is that it introduces a
couple of nested loops, where the out loop iterates from
i=0..nelts and the inner loop iterates from j=0..i.
To avoid performance concerns, a check is introduced that
ensures nelts won't exceed 4 lanes.

The added test case is derived from h264ref (the other cases from the
benchmark have the same structure and don't provide additional coverage).

Bootstrapped and regression-tested on x86-64 and aarch64.
Further, tested on CPU 2006 h264ref and CPU 2017 x264.

	PR117830

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (get_vect_selector_index_map): Removed.
	(recognise_vec_perm_simplify_seq): Fix calculation of vec-perm
	selectors of narrowed sequence.
	(calc_perm_vec_perm_simplify_seqs): Fixing calculation of
	vec-perm selectors of the blended sequence.
	(process_vec_perm_simplify_seq_list): Add whitespace to dump
	string to avoid bad formatted dump output.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vector-11.c: New test.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

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