[dev.ssa] cmd/compile: preallocate small-numbered values and blocks

Speeds up the compiler ~5%.

Change-Id: Ia5cf0bcd58701fd14018ec77d01f03d5c7d6385b
Reviewed-on: https://go-review.googlesource.com/19060
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/pgen.go

@@ -496,6 +496,7 @@ func compile(fn *Node) {
 		if Curfn.Func.Endlineno != 0 {
 			lineno = Curfn.Func.Endlineno
 		}
+		ssafn.Free()
 		return
 	}
 	Genlist(Curfn.Func.Enter)