When a println arg contains a call to an inlineable function
that itself contains a switch, that switch statement will be
walked twice, once by the walkexprlist formerly in the
OPRINT/OPRINTN case, then by walkexprlistcheap in walkprint.

Remove the first walkexprlist, it is not necessary.
walkexprlist =
		s[i] = walkexpr(s[i], init)
walkexprlistcheap = {
		s[i] = cheapexpr(n, init)
		s[i] = walkexpr(s[i], init)
}

Seems like this might be possible in other places, i.e.,
calls to inlineable switch-containing functions.

See also #25776.
Fixes #29220.

Change-Id: I3781e86aad6688711597b8bee9bc7ebd3af93601
Reviewed-on: https://go-review.googlesource.com/c/154497
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>