[release-branch.go1.8] cmd/compile/internal/syntax: avoid follow-up error for incorrect if statement

This is a follow-up on https://go-review.googlesource.com/36470
and leads to a more stable fix. The above CL relied on filtering
of multiple errors on the same line to avoid more than one error
for an `if` statement of the form `if a := 10 {}`. This CL avoids
the secondary error ("missing condition in if statement") in the
first place.

For #18915.

Change-Id: I8517f485cc2305965276c17d8f8797d61ef9e999
Reviewed-on: https://go-review.googlesource.com/36479
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-on: https://go-review.googlesource.com/36424
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>