tree-optimization/121256 - properly support SLP in vectorizable recurrence

tree-optimization/121256 - properly support SLP in vectorizable recurrence

We failed to build the correct initialization vector.  For VLA
vectors and a non-uniform initialization vector this rejects
vectorization for now.

	PR tree-optimization/121256
	* tree-vect-loop.cc (vectorizable_recurr): Build a correct
	initialization vector for SLP_TREE_LANES > 1.

	* gcc.dg/vect/vect-recurr-pr121256.c: New testcase.
	* gcc.dg/vect/vect-recurr-pr121256-2.c: Likewise.

(cherry picked from commit 26dc9aa285b53551c55d3d660bb6da21d59d7023)