Make dse.c use offset/width instead of start/end

Make dse.c use offset/width instead of start/end

store_info and read_info_type in dse.c represented the ranges as
start/end, but a lot of the internal code used offset/width instead.
Using offset/width throughout fits better with the poly_int.h
range-checking functions.

2017-12-15  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* dse.c (store_info, read_info_type): Replace begin and end with
	offset and width.
	(print_range): New function.
	(set_all_positions_unneeded, any_positions_needed_p)
	(check_mem_read_rtx, scan_stores, scan_reads, dse_step5): Update
	accordingly.
	(record_store): Likewise.  Optimize the case in which all positions
	are unneeded.
	(get_stored_val): Replace read_begin and read_end with read_offset
	and read_width.
	(replace_read): Update call accordingly.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255692