[testsuite] Fix f16_mov_immediate_3.c

Fix and simplify the testcase so it generates dup even on latest trunk.

    testsuite/
	* gcc.target/aarch64/f16_mov_immediate_3.c: Fix testcase.

From-SVN: r262222

gcc/testsuite/ChangeLog

+2018-06-28  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	* gcc.target/aarch64/f16_mov_immediate_3.c: Fix testcase.
+
 2018-06-28  Fritz Reese  <fritzoreese@gmail.com>
 
 	PR fortran/82865

gcc/testsuite/gcc.target/aarch64/f16_mov_immediate_3.c

 /* { dg-do compile } */
-/* { dg-options "-O0" } */
-
-extern __fp16 foo ();
+/* { dg-options "-O2" } */
 
 __fp16 f4 ()
 {
-  __fp16 a = 0;
-  __fp16 b = 1;
-  __fp16 c = 2;
-  __fp16 d = 4;
+  __fp16 a = 0.1;
 
-  __fp16 z = a + b;
-  z = z + c;
-  z = z - d;
+  __fp16 z = a * a;
   return z;
 }