These functions are really simple, the overhead of calling
them (in both time and code size) is larger than the inlined versions.

Reorganize how the nil case in a type switch is handled, as we have
to check for nil explicitly now anyway.

Saves about 0.8% in the binary size of the go tool.

Change-Id: I8501b62d72fde43650b79f52b5f699f1fbd0e7e7
Reviewed-on: https://go-review.googlesource.com/19814
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>