poly_int: decode_addr_const

poly_int: decode_addr_const

This patch makes the varasm-local addr_const track polynomial offsets.
I'm not sure how useful this is, but it was easier to convert than not.

2017-12-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* varasm.c (addr_const::offset): Change from HOST_WIDE_INT
	to poly_int64.
	(decode_addr_const): Update accordingly.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255927