cmd/internal/ssa: utility functions to make Funcs

cmd/internal/ssa: utility functions to make Funcs

Adds a more convenient way to define Funcs for testing.
For instance,

  b1:
    v1 = Arg <mem> [.mem]
    Plain -> b2
  b2:
    Exit v1
  b3:
    v2 = Const <bool> [true]
    If v2 -> b3 b2

can be defined as

	 fun :=Fun("entry",
		Bloc("entry",
			Valu("mem", OpArg, TypeMem, ".mem"),
			Goto("exit")),
		Bloc("exit",
			Exit("mem")),
		Bloc("deadblock",
			Valu("deadval", OpConst, TypeBool, true),
			If("deadval", "deadblock", "exit")))

Also add an Equiv function to test two Funcs for equivalence.

Change-Id: If1633865aeefb8e765e772b6dad19250d93a413a
Reviewed-on: https://go-review.googlesource.com/9992
Reviewed-by: Keith Randall <khr@golang.org>