all: join a few chained ifs

all: join a few chained ifs

I had been finding these over a year or so, but none were big enough
changes to warrant CLs. They're a handful now, so clean them all up in a
single commit.

The smaller bodies get a bit simpler, but most importantly, the larger
bodies get unindented.

Change-Id: I5707a6fee27d4c9ff9efd3d363af575d7a4bf2aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/165340
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>