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