Fix calculation of ptr_mode for MODE_PARTIAL_INT Pmode

This patch fixes a regression caused by r251469, where I'd incorrectly
converted a call to mode_for_size that sometimes needs MODE_PARTIAL_INTs.

2017-11-22  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* emit-rtl.c (init_derived_machine_modes): Make sure ptr_mode
	has the same mode class as Pmode.

From-SVN: r255057

gcc/ChangeLog

+2017-11-22  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* emit-rtl.c (init_derived_machine_modes): Make sure ptr_mode
+	has the same mode class as Pmode.
+
 2017-11-22  Thomas Preud'homme  <thomas.preudhomme@arm.com>
 
 	* config/arm/arm.c (cmse_nonsecure_call_clear_caller_saved): Get rid of

gcc/emit-rtl.c

@@ -6003,7 +6003,8 @@ init_derived_machine_modes (void)
 
   byte_mode = opt_byte_mode.require ();
   word_mode = opt_word_mode.require ();
-  ptr_mode = int_mode_for_size (POINTER_SIZE, 0).require ();
+  ptr_mode = as_a <scalar_int_mode>
+    (mode_for_size (POINTER_SIZE, GET_MODE_CLASS (Pmode), 0).require ());
 }
 
 /* Create some permanent unique rtl objects shared between all functions.  */