tree-optimization/92645 - improve SLP with existing vectors

tree-optimization/92645 - improve SLP with existing vectors

This improves SLP discovery in the face of existing vectors allowing
punning of the vector shape (or even punning from an integer type).
For punning from integer types this does not yet handle lane zero
extraction being represented as conversion rather than BIT_FIELD_REF.

2021-01-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/92645
	* tree-vect-slp.c (vect_build_slp_tree_1): Relax supported
	BIT_FIELD_REF argument.
	(vect_build_slp_tree_2): Record the desired vector type
	on the external vector def.
	(vectorizable_slp_permutation): Handle required punning
	of existing vector defs.

	* gcc.target/i386/pr92645-6.c: New testcase.