Minor vn_reference_lookup_3 tweak

Minor vn_reference_lookup_3 tweak

The repeated checks for MEM_REF made this code hard to convert to
poly_ints as-is.  Hopefully the new structure also makes it clearer
at a glance what the two cases are.

2017-12-16  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-ssa-sccvn.c (vn_reference_lookup_3): Avoid repeated
	checks for MEM_REF.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255742