cmd/compile: move Node.Walkdef into flags

cmd/compile: move Node.Walkdef into flags

Node.Walkdef is 0, 1, or 2, so it only requires two bits.
Add support for 2-bit values to bitset,
and use it for Node.Walkdef.

Class, Embedded, Typecheck, and Initorder will follow suit
in subsequent CLs.

The multi-bit flags will go at the beginning,
since that generates (marginally) more efficient code.

Change-Id: Id6e2e66e437f10aaa05b8a6e1652efb327d06128
Reviewed-on: https://go-review.googlesource.com/41791
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>