runtime: calculate k only once in mapiternext

runtime: calculate k only once in mapiternext

Make the calculation of k and v a bit lazier.
None of the following code cares about indirect-vs-direct k,
and it happens on all code paths, so check t.indirectkey earlier.

Simplifies the code and reduces both machine code and stack size.

Change-Id: I5ea4c0772848d7a4b15383baedb9a1f7feb47201
Reviewed-on: https://go-review.googlesource.com/55092
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>