Simplify vec_merge according to the mask.

Simplify vec_merge according to the mask.

This patch was part of the original patch we acquired from Honza and Martin.

It simplifies nested vec_merge operations using the same mask.

Self-tests are included.

2018-09-28  Andrew Stubbs  <ams@codesourcery.com>
	    Jan Hubicka  <jh@suse.cz>
	    Martin Jambor  <mjambor@suse.cz>

	* simplify-rtx.c (simplify_merge_mask): New function.
	(simplify_ternary_operation): Use it, also see if VEC_MERGEs with the
	same masks are used in op1 or op2.
	(test_vec_merge): New function.
	(test_vector_ops): Call test_vec_merge.

Co-Authored-By: Jan Hubicka <jh@suse.cz>
Co-Authored-By: Martin Jambor <mjambor@suse.cz>
From-SVN: r264688