poly_int: find_bswap_or_nop_load

poly_int: find_bswap_or_nop_load

This patch handles polynomial offsets in find_bswap_or_nop_load,
which could be useful for constant-sized data at a variable offset.
It is needed for a later patch to compile.

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

gcc/
	* gimple-ssa-stor-merging.c (find_bswap_or_nop_load): Track polynomial
	offsets for MEM_REFs.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255929