Make more use of subreg_lowpart_offset

Make more use of subreg_lowpart_offset

This patch uses subreg_lowpart_offset in places that open-coded
the calculation.  It also uses it in regcprop.c to test whether,
after a mode change, the first register in a multi-register group
is still the right one.

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

gcc/
	* calls.c (expand_call): Use subreg_lowpart_offset.
	* cse.c (cse_insn): Likewise.
	* regcprop.c (copy_value): Likewise.
	(copyprop_hardreg_forward_1): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r253713