Make more use of subreg_size_lowpart_offset

Make more use of subreg_size_lowpart_offset

This patch uses subreg_size_lowpart_offset in places that open-coded
the calculation.  The reload use (and the LRA one that was based on it)
seemed to ignore the BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN case; it's not
obvious whether that was deliberate or an oversight.

2017-10-22  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* lra-spills.c (assign_mem_slot): Use subreg_size_lowpart_offset.
	* regcprop.c (maybe_mode_change): Likewise.
	* reload1.c (alter_reg): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r253989