Support group size of three in SLP store permute lowering

The following implements the group-size three scheme from
vect_permute_store_chain in SLP grouped store permute lowering
and extends it to power-of-two multiples of group size three.

The scheme goes from vectors A, B and C to
{ A[0], B[0], C[0], A[1], B[1], C[1], ... } by first producing
{ A[0], B[0], X, A[1], B[1], X, ... } (with X random but chosen
to A[n]) and then permuting in C[n] in the appropriate places.

The extension goes as to replace vector elements with a
power-of-two number of lanes and you'd get pairwise interleaving
until the final three input permutes happen.

The last permute step could be seen as extending C to { C[0], C[0],
C[0], ... } and then performing a blend.

VLA archs will want to use store-lanes here I guess, I'm not sure
if the three vector interleave operation is also available with
a register source and destination and thus available for a shuffle.

	* tree-vect-slp.cc (vect_build_slp_instance): Special case
	three input permute with the same number of lanes in store
	permute lowering.

	* gcc.dg/vect/slp-53.c: New testcase.
	* gcc.dg/vect/slp-54.c: New testcase.