large compile times for mlyacc

[Matthew Fluet]

> I am seeing large compile times when compiling mlyacc.  The problem
> appears to be in generateTransfers of the x86codegen, which is taking
> about 1/2 of the total compile time.  Matthew, any idea what's going
> on?

Yeah; look at the cvs log for directed-graph.sml.

Unfortunately, rolling back didn't fix the problem, although I thought for
sure I had run self compiles with the codegen using the loopForest stuff.

Choices are:
 1. somebody look at loopForest and figure out how to speed up nest
 2. forget about using loopForest info in codegen
 3. (stopgap) the codegen should work o.k. if x86LoopInfo.createLoopInfo
     returns information that makes it look like no block is part of a
     loop (the codegen only uses to weight stuff carried in registers
     and to insert alignment at loop headers).

I can probably cobble 3 together in a little while.