[dev.ssa] cmd/internal/gc: convert standard IR into SSA.

Hook into the current compiler to convert the existing
IR (after walk) into SSA.  Any function ending in "_ssa"
will take this path.  The resulting assembly is printed
and then discarded.

Use gc.Type directly in ssa instead of a wrapper for go types.
It makes the IR->SSA rewrite a lot simpler.

Only a few opcodes are implemented in this change.  It is
enough to compile simple examples like
    func f(p *int) int { return *p }
    func g(a []int, i int) int { return a[i] }

Change-Id: I5e18841b752a83ca0519aa1b2d36ef02ce1de6f9
Reviewed-on: https://go-review.googlesource.com/8971
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/internal/ssa/cse.go

@@ -4,9 +4,7 @@
 
 package ssa
 
-import (
-	"sort"
-)
+import "sort"
 
 // cse does common-subexpression elimination on the Function.
 // Values are just relinked, nothing is deleted.  A subsequent deadcode
@@ -115,7 +113,9 @@ func cse(f *Func) {
 			// Replace all elements of e which v dominates
 			for i := 0; i < len(e); {
 				w := e[i]
-				if w != v && dom(v.Block, w.Block, idom) {
+				if w == v {
+					e, e[i] = e[:len(e)-1], e[len(e)-1]
+				} else if dom(v.Block, w.Block, idom) {
 					rewrite[w.ID] = v
 					e, e[i] = e[:len(e)-1], e[len(e)-1]
 				} else {