From a compact specification of control flow graphs,
generate complete set of possible assignment patterns to
output y, and also generate an interpretable specification.
Compiles (hoping for crash, or not) and then runs, where
the run checks function output against interpreted output
for various inputs observed to terminate in the interpreter.

In ssa_test.go, added ability to generate a test and run
(compile and run) the generated test, possibly with
modified environment variables.  The generated test is
compiled including the -D=ssa/check/on flag, and if the
interpreter terminates in a small number of steps, then it
is also run to check the result.

Change-Id: I392c828e36c543411b7733ca0799628452733276
Reviewed-on: https://go-review.googlesource.com/22751
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>