Use MEM_SIZE rather than GET_MODE_SIZE in dce.c

Use MEM_SIZE rather than GET_MODE_SIZE in dce.c

Using MEM_SIZE is more general, since it copes with cases where
targets are forced to use BLKmode references for whatever reason.

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

	* dce.c (check_argument_store): Pass the size instead of
	the memory reference.
	(find_call_stack_args): Pass MEM_SIZE to check_argument_store.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@242446 138bc75d-0d04-0410-961f-82ee72b054a4