verify_expr ensured that the size and offset in gimple BIT_FIELD_REFs
satisfied tree_fits_uhwi_p.  This patch extends that so that they can
be poly_uint64s, and adds helper routines for accessing them when the
verify_expr requirements apply.

2017-12-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree.h (bit_field_size, bit_field_offset): New functions.
	* hsa-gen.c (gen_hsa_addr): Use them.
	* tree-ssa-forwprop.c (simplify_bitfield_ref): Likewise.
	(simplify_vector_constructor): Likewise.
	* tree-ssa-sccvn.c (copy_reference_ops_from_ref): Likewise.
	* tree-cfg.c (verify_expr): Require the sizes and offsets of a
	BIT_FIELD_REF to be poly_uint64s rather than uhwis.
	* fold-const.c (fold_ternary_loc): Protect tree_to_uhwi with
	tree_fits_uhwi_p.

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

From-SVN: r255926