[dev.ssa] cmd/compile/ssa: print lazily

Reduces 'go run run.go 64bit.go' from 23s to 8s on my machine.

Change-Id: Ie5b642d0abb56e8eb3899d69472bc88a85a1c985
Reviewed-on: https://go-review.googlesource.com/12023Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/print.go

@@ -11,7 +11,7 @@ import (
 )
 
 func printFunc(f *Func) {
-	f.Logf("%s", f.String())
+	f.Logf("%s", f)
 }
 
 func (f *Func) String() string {