self-compile

[Matthew Fluet]

What's the memory requirement for a G2 self-compile?  I was trying to do
one from an x86-G1 and it was spending all of its time paging at the
"finish infer" phase.  The heap usage should be identical between the two
backends, but I haven't tried using a C-G1.

In other news, with some other optimizations from yesterday, the
x86-codegen is winning in all benchmarks except checksum.  (Yes, this
includes life and wc.)  I'll send out some hard numbers later today -- I'm
trying to track down why I lost some performance on fib and tak, which
were previously the best improvments.  My guess is that it is an artifact
of only committing live pseudo-regs down their respective branches.  I
lift out all pseudo-regs that are live down both, and then make the fall
through case be the branch with the most remaining live pseudo-regs.  This
might reverse some branches and screw up the branch prediction.