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

Use distinction between explicit and automatically inserted semicolons
to provide a better error message if the condition in an 'if' statement
is missing.

For #18747.

Change-Id: Iac167ae4e5ad53d2dc73f746b4dee9912434bb59
Reviewed-on: https://go-review.googlesource.com/36930


Reviewed-by: Matthew Dempsky <mdempsky@google.com>