Add a loop fusion pass.

This pass will look for adjacent loops that are compatible and legal to
be fused.

Loops are compatible if:

- they both have one induction variable
- they have the same upper and lower bounds
    - same initial value
    - same condition
- they have the same update step
- they are adjacent
- there are no break/continue in either of them

Fusion is legal if:

- fused loops do not have any dependencies with dependence distance
  greater than 0 that did not exist in the original loops.
- there are no function calls in the loops (could have side-effects)
- there are no barriers in the loops

It will fuse all such loops as long as the number of registers used for
the fused loop stays under the threshold defined by
max_registers_per_loop.