compiler: pass lvalue/rvalue context to back end for var exprs

compiler: pass lvalue/rvalue context to back end for var exprs

    Add a new flag on the Var_expression class that indicates
    whether the var reference appears in an "lvalue" context
    (for example, on the LHS of an assignment stmt) or an
    "rvalue" context (for example, as an argument of a call).

    Add a traversal pass that visits assignment stmt LHS subtrees
    so as to mark things prior to backend gen. Select the right
    context value in other places where Backend::var_expression is
    called.

    Reviewed-on: https://go-review.googlesource.com/33990

	* go-gcc.cc (Gcc_backend::var_expression): Add Varexpr_context
	parameter.

From-SVN: r243321