runtime: skip wb call in growslice when unnecessary

runtime: skip wb call in growslice when unnecessary

Instrumenting make.bash reveals that almost half (49.54%)
of the >16 million calls to growslice for
pointer-containing slices are
growing from an empty to a non-empty slice.

In that case, there is no need to call the write barrier,
which does some work before discovering that no pointers need shading.

Change-Id: Ide741468d8dee7ad43ea0bfbea6ccdf680030a0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/168959
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>