Fix unguarded uses of tree_to_uhwi

Fix unguarded uses of tree_to_uhwi

This patch uses tree_fits_uhwi_p to protect a previously unguarded
use of tree_to_uhwi.  Previously we would ICE for variable-sized types.

2017-09-20  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree.c (find_atomic_core_type): Check tree_fits_uhwi_p before
	calling tree_to_uhwi.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r253057