runtime: unify cases in mapiternext

runtime: unify cases in mapiternext

The preceding cleanup made it clear that two cases
(have golden data, unreachable key) are handled identically.
Simplify the control flow to reflect that.

Simplifies the code and generates shorter machine code.

Change-Id: Id612e0da6679813e855506f47222c58ea6497d70
Reviewed-on: https://go-review.googlesource.com/55093
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>