cmd/compile/internal/syntax: remove need for missing_statement (fixed TODO)
cmd/compile/internal/syntax: remove need for missing_statement (fixed TODO)

Now that we have consistent use of xOrNil parse methods, we don't
need a special missing_statement singleton to distinguish between
missing actually statements and other errors (which have returned
nil before).

For #19663.

Change-Id: I8364f1441bdf8dd966bcd6d8219b2a42d6b88abd
Reviewed-on: https://go-review.googlesource.com/38656
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

Cofyc/go,golang/go,ImJasonH/go,Cofyc/go,josharian/go,codestation/go,polyverse-security/go,golang/go,christopher-henderson/Go,codestation/go,nawawi/go,nawawi/go,derekmarcotte/go,akutz/go,akutz/go,golang/go,linux-on-ibm-z/go,polyverse-security/go,linux-on-ibm-z/go,ImJasonH/go,akutz/go,Cofyc/go,Cofyc/go,nawawi/go,Cofyc/go,derekmarcotte/go,cloudflare/go,josharian/go,CAFxX/go,nawawi/go,ImJasonH/go,golang/go,codestation/go,derekmarcotte/go,Cofyc/go,polyverse-security/go,codestation/go,cloudflare/go,ImJasonH/go,akutz/go,nawawi/go,josharian/go,ImJasonH/go,codestation/go,akutz/go,Cofyc/go,polyverse-security/go,derekmarcotte/go,josharian/go,cloudflare/go,codestation/go,CAFxX/go,vsekhar/elastic-go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,codestation/go,linux-on-ibm-z/go,akutz/go,ImJasonH/go,CAFxX/go,ImJasonH/go,akutz/go,vsekhar/elastic-go,CAFxX/go,derekmarcotte/go,linux-on-ibm-z/go,CAFxX/go,josharian/go,vsekhar/elastic-go,cloudflare/go,cloudflare/go,vsekhar/elastic-go,CAFxX/go,linux-on-ibm-z/go,akutz/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,derekmarcotte/go,vsekhar/elastic-go,golang/go,linux-on-ibm-z/go,nawawi/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,vsekhar/elastic-go,polyverse-security/go,golang/go,CAFxX/go,golang/go,nawawi/go,josharian/go,linux-on-ibm-z/go,josharian/go,golang/go,Cofyc/go,cloudflare/go,CAFxX/go,cloudflare/go,codestation/go,cloudflare/go,ImJasonH/go,derekmarcotte/go