Improve load permutation lowering

Improve load permutation lowering

The following makes sure the emitted even/odd extraction scheme
follows one that ends up with actual trivial even/odd extract permutes.
When we choose a level 2 extract we generate { 0, 1, 4, 5, ... }
which for example the x86 backend doesn't recognize with just SSE
and QImode elements.  So this now follows what the non-SLP interleaving
code would do which is element granular even/odd extracts.

This resolves gcc.dg/vect/vect-strided[-a]-u8-i8-gap*.c FAILs with
--param vect-force-slp=1 on x86_64.

	* tree-vect-slp.cc (vect_lower_load_permutations): Prefer
	level 1 even/odd extracts.