limit check insertion

[Matthew Fluet]

> Matthew, I'll leave simple to you, and start on whichever of prodcons
> and mutex you haven't already started on, or both if you want.

Go ahead and take both of them.  


I was just finishing up some additional profiling hacks in machine IL. 
The profiling info displayed in -d 1 should always be labels that appear
in SSA.  This eliminates the problem with continuation stubs, handler
stubs, and case dispatch stubs getting machine IL names.  Continuation
stubs' and handler stubs' ticks get allocated with the "real" continuation
or handler block.  Cases are a little trickier.  The final stubs that go
to the label mentioned in SSA are associated with the SSA label.  You can
also have additional stubs that split on int/pointer, etc.; these stubs'
ticks get allocated to the SSA block that has the case.