middle-end: optimize slp simplify back to back permutes.

middle-end: optimize slp simplify back to back permutes.

This optimizes sequential permutes. i.e. if there are two permutes back to back
this function applies the permute of the parent to the child and removed the
parent.

This relies on the materialization point calculation in optimize SLP.

This allows us to remove useless permutes such as

	ldr     q0, [x0, x3]
	ldr     q2, [x1, x3]
	trn1    v1.4s, v0.4s, v0.4s
	trn2    v0.4s, v0.4s, v0.4s
	trn1    v0.4s, v1.4s, v0.4s
	mov     v1.16b, v3.16b
	fcmla   v1.4s, v0.4s, v2.4s, #0
	fcmla   v1.4s, v0.4s, v2.4s, #90
	str     q1, [x2, x3]

from the sequence the vectorizer puts out and give

	ldr     q0, [x0, x3]
	ldr     q2, [x1, x3]
	mov     v1.16b, v3.16b
	fcmla   v1.4s, v0.4s, v2.4s, #0
	fcmla   v1.4s, v0.4s, v2.4s, #90
	str     q1, [x2, x3]

instead.

gcc/ChangeLog:

	* tree-vect-slp.c (vect_slp_tree_permute_noop_p): New.
	(vect_optimize_slp): Optimize permutes.
	(vectorizable_slp_permutation): Fix typo.