Conditions in Iterator IR

[havogt]

In if_(cond, true_branch, false_branch), both branches true_branch and false_branch will be evaluated eagerly (by construction). This can create out-of-bounds accesses. Therefore we have to pass the branches as lamdas. We need to check that lowering respects that and that type inference works for functions in true_branch and false_branch.

[havogt]

Discussion with @egparedes and @tehrengruber:

• for semantics of itir it doesn't matter what the evaluation order is, but in practice it matters for embedded execution and C++ execution
• for embedded execution we can work around by returning whatever (in out of bounds, e.g. NaN) but that might be very slow
• we could put unevaluated lambdas in an if_ to make them lazy, but
• for C++ we cannot do that because the lambda's in the 2 branches have different type
• therefore it seems the only option is to make if_ special in that only the 2nd or 3rd argument must be evaluated.

Other perspective on that problem
We can choose to leave evaluation order open, but then passes that change behavior under a certain evaluation order are not generally applicable. -> 2 choices: leave open and clearly state in each pass under which evaluation order they can be used (which means for which backend) or we fix evaluation order and all backends have to implement the same.