SSA backend update

[Matthew Fluet]

> Right now, I'm trying to understand why
> regression test 16 works fine if you run it without redirection, but fails when
> rredirecting to a file.  The failure mode is to output nothing and exit with
> status 0.  As best as I can tell with gdb, flush is confused and never calls
> Posix_IO_write.  I'll look into it on Monday.

I ran into this before.  There is a difference -- there is a call to
Posix_ProcEnv_isatty that differs with screen and file output.