[simplify-rtx] Simplify vec_merge of vec_duplicates into vec_concat

Another vec_merge simplification that's missing from simplify-rtx.c is transforming
a vec_merge of two vec_duplicates. For example:
(set (reg:V2DF 80)
    (vec_merge:V2DF (vec_duplicate:V2DF (reg:DF 84))
        (vec_duplicate:V2DF (reg:DF 81))
        (const_int 2)))

Can be transformed into the simpler:
(set (reg:V2DF 80)
    (vec_concat:V2DF (reg:DF 81)
                (reg:DF 84)))

I believe this should always be beneficial.
I'm still looking into finding a small testcase demonstrating this, but on aarch64 SPEC
I've seen this eliminate some really bizzare codegen where GCC was generating nonsense like:
  ldr q18, [sp, 448]
  ins v18.d[0], v23.d[0]
  ins v18.d[1], v22.d[0]

With q18 being pushed and popped off the stack in the prologue and epilogue of the function!
These are large files from SPEC that I haven't been able to analyse yet as to why GCC even attempts
to do that, but with this patch it doesn't try to load a register and overwrite all its lanes.
This patch shaves off about 5k of code size from zeusmp on aarch64 at -O3, so I believe it's a good
thing to do. 

	* simplify-rtx.c (simplify_ternary_operation): Simplify vec_merge
	of two vec_duplicates into a vec_concat.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254550 138bc75d-0d04-0410-961f-82ee72b054a4

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-08  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+	* simplify-rtx.c (simplify_ternary_operation): Simplify vec_merge
+	of two vec_duplicates into a vec_concat.
+
 2017-11-08  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
 
 	* simplify-rtx.c (simplify_ternary_operation, VEC_MERGE):

gcc/simplify-rtx.c

@@ -5784,6 +5784,24 @@ simplify_ternary_operation (enum rtx_code code, machine_mode mode,
 	      if (!side_effects_p (otherop))
 		return simplify_gen_binary (VEC_CONCAT, mode, newop0, newop1);
 	    }
+
+	  /* Replace (vec_merge (vec_duplicate x) (vec_duplicate y)
+				 (const_int n))
+	     with (vec_concat x y) or (vec_concat y x) depending on value
+	     of N.  */
+	  if (GET_CODE (op0) == VEC_DUPLICATE
+	      && GET_CODE (op1) == VEC_DUPLICATE
+	      && GET_MODE_NUNITS (GET_MODE (op0)) == 2
+	      && GET_MODE_NUNITS (GET_MODE (op1)) == 2
+	      && IN_RANGE (sel, 1, 2))
+	    {
+	      rtx newop0 = XEXP (op0, 0);
+	      rtx newop1 = XEXP (op1, 0);
+	      if (sel == 2)
+		std::swap (newop0, newop1);
+
+	      return simplify_gen_binary (VEC_CONCAT, mode, newop0, newop1);
+	    }
 	}
 
       if (rtx_equal_p (op0, op1)