Add an optimization to convert set_subtensor() of an add into inc_subtensor().

This did come up in an implementation of adagrad at some point and will be much faster on GPU.
Add a test that the optimization is applied and works.
Add comment as per review.
Fix test in FAST_COMPILE.
Merge pull request #2338 from abergeron/set_to_inc

Convert SetSubtensor to IncSubtensor when possible