Print emitted merge tree

[fredrikbk]

Print the merge tree of expressions that are recursively emitted for different merge cases.

For example, the merge tree for DCSR matrix addition is:

forall(i, forall(j, A(i,j) = B(i,j) + C(i,j)))
    forall(j, A(i,j) = B(i,j) + C(i,j))
        A(i,j) = B(i,j) + C(i,j)
        A(i,j) = B(i,j)
        A(i,j) = C(i,j)
    forall(j, A(i,j) = B(i,j))
        A(i,j) = B(i,j)
    forall(j, A(i,j) = C(i,j))
        A(i,j) = C(i,j)