tree-optimization/109735 - conversion for vectorized pointer-diff

tree-optimization/109735 - conversion for vectorized pointer-diff

There's handling in vectorizable_operation for POINTER_DIFF_EXPR
requiring conversion of the result of the unsigned operation to
a signed type.  But that's conditional on the "default" kind of
vectorization.  In this PR it's shown the emulated vector path
needs it and I think the masked operation case will, too (though
we might eventually never mask an integral MINUS_EXPR).  So the
following makes that handling unconditional.

	PR tree-optimization/109735
	* tree-vect-stmts.cc (vectorizable_operation): Perform
	conversion for POINTER_DIFF_EXPR unconditionally.