poly_int: memrefs_conflict_p

The xsize and ysize arguments to memrefs_conflict_p are encode such
that:

- 0 means the size is unknown
- >0 means the size is known
- <0 means that the negative of the size is a worst-case size after
  alignment

In other words, the sign effectively encodes a boolean; it isn't
meant to be taken literally.  With poly_ints these correspond to:

- must_eq (..., 0)
- may_gt (..., 0)
- may_lt (..., 0)

respectively.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* alias.c (addr_side_effect_eval): Take the size as a poly_int64
	rather than an int.  Use plus_constant.
	(memrefs_conflict_p): Take the sizes as poly_int64s rather than ints.
	Take the offset "c" as a poly_int64 rather than a HOST_WIDE_INT.

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

From-SVN: r256163