Differential returns that are pointer types

[timkaler]

We currently do not seem to handle differential returns that are pointer types. e.g. a call %x = float* foo() where %x is not a constant value. A test case for this is attached --- if you look at the test case please spend a moment to verify that I set it up correctly as it was extracted from a larger file generated from eigen code. If the general fix isn't easy we should discuss exactly what we want to do here (e.g. create a wrapper function for such calls that put all of our differentiable calls into a common form where certain return values are additional arguments)

eigen-small-test.ll.txt

[wsmoses]

This should be handled (and there are some small test cases for) — will look into shortly, this is an implementation bug

[wsmoses]

Already fixed this with more minimal test case (will push later tonight because travel).

[timkaler]

Already fixed this with more minimal test case (will push later tonight because travel).

Great, if you have the opportunity to make a pull request without too much inconvenience it would be helpful. This case is blocking attempts to find more implementation-gaps/bugs, and it's a case that I'd like to make sure is handled correctly by the load cache elision logic, and in the active/inactive detection logic.

[wsmoses]

Pushed #27