This allows the compiler to generate better code
containing fewer jumps and only a single return value.

Cuts 12k off cmd/go and 16k off golang.org/x/tools/cmd/godoc, approx 0.1% each.

For #6853 and #9930

Change-Id: I009616df797760b01e09f06357a2d6fd6ebcf307
Reviewed-on: https://go-review.googlesource.com/19767Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>