utils2.c (known_alignment): Derive the alignment from pointed-to types only if it is otherwise unknown.

	* utils2.c (known_alignment): Derive the alignment from pointed-to
	types only if it is otherwise unknown.
	<INTEGER_CST>: Tidy.
	<MULT_EXPR>: Likewise.
	<POINTER_PLUS_EXPR>: If the alignment of the offset is unknown, use
	that of the base.

From-SVN: r137081