Add depth=-1 mode to air-par-to-herd/segment/launch

[erwei-xilinx]

… representing converting the innermost loop body into air.hierarchy op.

Assigning other negative values to depth means converting all loops; it would be ambiguous to assign depth=-2 as being the second innermost body, because that would be ambiguous.

[fifield]

This seems like a useful shortcut vs. knowing the target's loop depth. The "any other negative" is the previous behavior? i.e. the option was initialized to -1 before

[erwei-xilinx]

This seems like a useful shortcut vs. knowing the target's loop depth. The "any other negative" is the previous behavior? i.e. the option was initialized to -1 before

Yes, in the previous version, any negative value means the same, i.e. converting all parallel ops. And that used to be the default behaviour.