This patch makes operand_subword and operand_subword_force take
polynomial offsets.  This is a fairly old-school interface and
these days should only be used when splitting multiword operations
into word operations.  It still doesn't hurt to support polynomial
offsets and it helps make callers easier to write.

2017-12-20  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* rtl.h (operand_subword, operand_subword_force): Take the offset
	as a poly_uint64 an unsigned int.
	* emit-rtl.c (operand_subword, operand_subword_force): Likewise.

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

From-SVN: r255883