[obvious] Use std::swap instead of manually swapping in a few more places

[obvious] Use std::swap instead of manually swapping in a few more places

	* alias.c (nonoverlapping_memrefs_p): Use std::swap instead of
	manually swapping values.
	* cse.c (fold_rtx): Likewise.
	* lra-eliminations.c (form_sum): Likewise.

From-SVN: r226179