cmd/compile/internal/syntax: better error msg for some 'if' statements

cmd/compile/internal/syntax: better error msg for some 'if' statements

R=go1.11

A common error is to write '=' instead of '==' inside the condition
of a simple 'if' statement:

	if x = 0 { ... }

Highlight the fact that we have an assignment in the error message
to prevent further confusion.

Fixes #23385.

Change-Id: I1552050fd6da927bd12a1be0977bd2e98eca5885
Reviewed-on: https://go-review.googlesource.com/87316
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>