Do not replace assign LHS in gtCloneExpr

Do not replace assign LHS in gtCloneExpr

Split the var/val parameters into two sets -- one for the current level of
recursion, the other for deeper levels.  Use this to suppress one level of
replacement when recursing to the LHS of an assign.  This both avoids
creating nonsensical assign-to-immediate trees and allows replacing uses
of the given var while leaving defs in the code.