[AArch64] Fix fp16 test failures after PR82964 fix

This fixes test failures in gcc.target/aarch64/f16_mov_immediate_*.c
after fixing PR82964.  The check for a scalar floating point constant
didn't include 16-bit floating point modes, so use GET_MODE_CLASS
instead.

    gcc/
	PR target/82964
	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p):
	Use GET_MODE_CLASS for scalar floating point.

From-SVN: r256854

gcc/ChangeLog

+2018-01-18  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	PR target/82964
+	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p):
+	Use GET_MODE_CLASS for scalar floating point.
+
 2018-01-18  Jan Hubicka  <hubicka@ucw.cz>
 
 	PR ipa/82256

gcc/config/aarch64/aarch64.c

@@ -11757,8 +11757,7 @@ aarch64_legitimate_constant_p (machine_mode mode, rtx x)
 {
   /* Support CSE and rematerialization of common constants.  */
   if (CONST_INT_P (x)
-      || (CONST_DOUBLE_P (x)
-	  && (mode == SFmode || mode == DFmode || mode == TFmode))
+      || (CONST_DOUBLE_P (x) && GET_MODE_CLASS (mode) == MODE_FLOAT)
       || GET_CODE (x) == CONST_VECTOR)
     return true;