First pass adding code for SSA backend. It is standalone for now.
I've included just a few passes to make the review size manageable -
I have more passes coming.
cmd/internal/ssa is the library containing the ssa compiler proper.
cmd/internal/ssa/ssac is a driver that loads an sexpr-based IR,
converts it to SSA form, and calls the above library. It is essentially
throwaway code - it will disappear once the Go compiler calls
cmd/internal/ssa itself. The .goir files in ssac/ are dumps of fibonacci
programs I made from a hacked-up compiler. They are just for testing.
Change-Id: I5ee89356ec12c87cd916681097cd3c2cd591040c
Reviewed-on: https://go-review.googlesource.com/6681
Reviewed-by: Alan Donovan <adonovan@google.com>
