Fix a GET_MODE_CLASS typo in mem_loc_descriptor

It should have been checking the size instead.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* dwarf2out.c (mem_loc_descriptor): Fix GET_MODE_CLASS/
	GET_MODE_SIZE typo.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242409