poly_int: brig vector elements

This patch adds a brig-specific wrapper around TYPE_VECTOR_SUBPARTS,
since presumably it will never need to support variable vector lengths.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/brig/
	* brigfrontend/brig-util.h (gccbrig_type_vector_subparts): New
	function.
	* brigfrontend/brig-basic-inst-handler.cc
	(brig_basic_inst_handler::build_shuffle): Use it instead of
	TYPE_VECTOR_SUBPARTS.
	(brig_basic_inst_handler::build_unpack): Likewise.
	(brig_basic_inst_handler::build_pack): Likewise.
	(brig_basic_inst_handler::build_unpack_lo_or_hi): Likewise.
	(brig_basic_inst_handler::operator ()): Likewise.
	(brig_basic_inst_handler::build_lower_element_broadcast): Likewise.
	* brigfrontend/brig-code-entry-handler.cc
	(brig_code_entry_handler::get_tree_cst_for_hsa_operand): Likewise.
	(brig_code_entry_handler::get_comparison_result_type): Likewise.
	(brig_code_entry_handler::expand_or_call_builtin): Likewise.

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

From-SVN: r256144