Fix missed CSE with a BLKmode entity

The following fixes fallout of r10-7145-g1dc00a8ec9aeba which made
us cautionous about CSEing a load to an object that has padding bits.
The added check also triggers for BLKmode entities like STRING_CSTs
but by definition a BLKmode entity does not have padding bits.

	PR tree-optimization/111751
	* tree-ssa-sccvn.cc (visit_reference_op_load): Exempt
	BLKmode result from the padding bits check.