[32/77] Check is_a <scalar_int_mode> before calling valid_pointer_mode

[32/77] Check is_a <scalar_int_mode> before calling valid_pointer_mode

A future patch will make valid_pointer_mode take a scalar_int_mode
instead of a machine_mode.  is_a <...> rather than as_a <...> is
needed here because we're checking a mode supplied by the user.

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/c-family/
	* c-attribs.c (handle_mode_attribute): Check for a scalar_int_mode
	before calling targetm.addr_space.valid_pointer_mode.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251484