Simplify subreg of vec_merge of vec_duplicate

Simplify subreg of vec_merge of vec_duplicate

We can simplify

  (subreg (vec_merge (vec_duplicate X)
		     (vector)
		     (const_int ((1 << N) | M)))
	  (N * sizeof (X)))

to X when mode of X is the same as of mode of subreg.

gcc/

	PR target/87537
	* simplify-rtx.c (simplify_subreg): Simplify subreg of vec_merge
	of vec_duplicate.
	(test_vector_ops_duplicate): Add test for a scalar subreg of a
	VEC_MERGE of a VEC_DUPLICATE.

gcc/testsuite/

	PR target/87537
	* gcc.target/i386/pr87537-1.c: New test.

From-SVN: r265260