tree-optimization/111125 - avoid BB vectorization in novector loops

tree-optimization/111125 - avoid BB vectorization in novector loops

When a loop is marked with

  #pragma GCC novector

the following makes sure to also skip BB vectorization for contained
blocks.  That avoids gcc.dg/vect/bb-slp-29.c failing on aarch64
because of extra BB vectorization therein.  I'm not specifically
dealing with sub-loops of novector loops, the desired semantics
isn't documented.

	PR tree-optimization/111125
	* tree-vect-slp.cc (vect_slp_function): Split at novector
	loop entry, do not push blocks in novector loops.

(cherry picked from commit 43da77a4f1636280c4259402c9c2c543e6ec6c0b)