vect: Avoid duplicate_and_interleave for uniform vectors [PR112661]

vect: Avoid duplicate_and_interleave for uniform vectors [PR112661]

can_duplicate_and_interleave_p checks whether we know a way of
building a particular VLA SLP invariant.  g:60034ecf25597bd515f
skipped that test for booleans, to support MASK_LEN_GATHER_LOAD
calls with a dummy all-ones mask.  But there's nothing fundamentally
different about VLA masks vs VLA data vectors.  If we have a VLA mask
that isn't all-ones, we need some way of loading it.  This ultimately
led to the ICE in the PR.

This patch fixes it by applying can_duplicate_and_interleave_p
to masks, while also adding a special path for uniform vectors
(of all kinds) to support the MASK_LEN_GATHER_LOAD usage.  This
also fixes an XFAIL in pr36648.cc for SVE.

The patch is mostly Richard's.  My only changes were to skip
redundant conversions and to use gimple_build_vector_from_val
for all eligible vectors.

2023-11-27  Richard Biener  <rguenther@suse.de>
	    Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/112661
	* tree-vect-slp.cc (vect_get_and_check_slp_defs): Defer duplicate-and-
	interleave test to...
	(vect_build_slp_tree_2): ...here, once we have all the operands.
	Skip the test for uniform vectors.
	(vect_create_constant_vectors): Detect uniform vectors.  Avoid
	redundant conversions in that case.  Use gimple_build_vector_from_val
	to build the vector.

gcc/testsuite/
	* g++.dg/vect/pr36648.cc: Remove XFAIL for VLA load-lanes.