Add trivial shortcuts in expair plumbing of class add.

Add trivial shortcuts in expair plumbing of class add.

These little shortcuts in add::split_ex_to_pair(ex) and in
add::combine_ex_with_coeff_to_pair(ex, ex) avoid the creation of
new ex objects. These pieces of code are executed so often that this
patch speeds up GiNaC by 5-10%, depending on test.