Corner case on replacements

[rckirby]

If the F given to the UFL substitutions indexes into gradients of components of a mixed space, the replacement process is broken. This shows up if one doesn't directly index, but takes the action of a derivative of a form that includes gradients, say for a linearly implicit IMEX splitting in Navier-Stokes.

It's possible (not checked) that this also breaks indexing into vector components of pieces of a mixed space.