This function accounts for 2% of "godoc -analysis=pointer"
and this change makes it twice as fast---and simpler.
Added test and benchmark.
Change-Id: I8578fa42dce34df057d81f6c522a7b4e0506d09d
Reviewed-on: https://go-review.googlesource.com/15211
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
