[dev.ssa] cmd/internal/ssa: SSA backend compiler skeleton

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>

src/cmd/internal/ssa/compile.go

@@ -0,0 +1,65 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssa
+
+import "fmt"
+
+// Compile is the main entry point for this package.
+// Compile modifies f so that on return:
+//   · all Values in f map to 0 or 1 assembly instructions of the target architecture
+//   · the order of f.Blocks is the order to emit the Blocks
+//   · the order of b.Values is the order to emit the Values in each Block
+//   · f has a non-nil regAlloc field
+func Compile(f *Func) {
+	// TODO: debugging - set flags to control verbosity of compiler,
+	// which phases to dump IR before/after, etc.
+	fmt.Printf("compiling %s\n", f.Name)
+
+	// hook to print function & phase if panic happens
+	phaseName := "init"
+	defer func() {
+		if phaseName != "" {
+			fmt.Printf("panic during %s while compiling %s\n", phaseName, f.Name)
+		}
+	}()
+
+	// Run all the passes
+	printFunc(f)
+	checkFunc(f)
+	for _, p := range passes {
+		phaseName = p.name
+		fmt.Printf("  pass %s begin\n", p.name)
+		p.fn(f)
+		fmt.Printf("  pass %s end\n", p.name)
+		printFunc(f)
+		checkFunc(f)
+	}
+
+	// Squash error printing defer
+	phaseName = ""
+}
+
+type pass struct {
+	name string
+	fn   func(*Func)
+}
+
+// list of passes for the compiler
+var passes = [...]pass{
+	{"phielim", phielim},
+	{"copyelim", copyelim},
+	//{"opt", opt},
+	// cse
+	{"deadcode", deadcode},
+	//{"fuse", fuse},
+	//{"lower", lower},
+	// cse
+	//{"critical", critical}, // remove critical edges
+	//{"layout", layout},     // schedule blocks
+	//{"schedule", schedule}, // schedule values
+	// regalloc
+	// stack slot alloc (+size stack frame)
+	//{"cgen", cgen},
+}