cmd/compile/internal/syntax: make a parser error "1.7 compliant"

cmd/compile/internal/syntax: make a parser error "1.7 compliant"

For code such as

	if a := 10 { ...

the 1.7 compiler reported

	a := 10 used as value

while the 1.8 compiler reported

	invalid condition, tag, or type switch guard

Changed the error message to match the 1.7 compiler.

Fixes #18915.

Change-Id: I01308862e461922e717f9f8295a9db53d5a914eb
Reviewed-on: https://go-review.googlesource.com/36470
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>