Make more use of byte_lowpart_offset

Make more use of byte_lowpart_offset

This patch uses byte_lowpart_offset in places that open-coded the
calculation.

2017-10-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* caller-save.c (replace_reg_with_saved_mem): Use byte_lowpart_offset.
	* combine.c (gen_lowpart_for_combine): Likewise.
	* dwarf2out.c (rtl_for_decl_location): Likewise.
	* final.c (alter_subreg): Likewise.
	* rtlhooks.c (gen_lowpart_general): Likewise.
	(gen_lowpart_if_possible): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r253714