Fix HWI + -unsigned in combine.c

Fix HWI + -unsigned in combine.c

rtx_equal_for_field_assignment_p had:

 	x = adjust_address_nv (x, GET_MODE (y),
			       -subreg_lowpart_offset (GET_MODE (x),
						       GET_MODE (y)));

But subreg_lowpart_offset returns an unsigned int and
adjust_address_nv takes a HWI, so a subreg offset of 4 would
give a memory offset of 0x00000000fffffffffc.

2017-10-23  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* combine.c (rtx_equal_for_field_assignment_p): Use
	byte_lowpart_offset.

From-SVN: r253997