Fixes many testsuite failures for SVE.

2018-08-28  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-ssa-sccvn.c (fully_constant_vn_reference_p): Fix unguarded
	use of tree_to_shwi.  Remove duplicated test for the size being
	a whole number of bytes.

From-SVN: r263914