runtime: make nextFreeFast inlinable

runtime: make nextFreeFast inlinable

https://golang.org/cl/22598 made nextFreeFast inlinable.
But during https://golang.org/cl/63611 it was discovered, that it is no longer inlinable.
Reduce number of statements below inlining threshold to make it inlinable again.
Also update tests, to prevent regressions.
Doesn't reduce readability.

Change-Id: Ia672784dd48ed3b1ab46e390132f1094fe453de5
Reviewed-on: https://go-review.googlesource.com/65030
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>