dashboard: enable linux/loong64 builder on 1.19 release branches

It made sense for the linux/loong64 builder to only test the main branch
during the port's development. Now that it's complete, update the policy
to test all supported Go versions (starting with Go 1.19, when the port
was added) instead of just tip.

This will increase the volume of work for the builder by some amount,
hopefully that's fine, it seems to have enough resources to handle it.

Updates golang/go#54222.

Change-Id: Ic774b9890fc4826e56613f360d6edd03f50e3deb
Reviewed-on: https://go-review.googlesource.com/c/build/+/459876
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2833,8 +2833,10 @@ func init() {
 		},
 		buildsRepo: func(repo, branch, goBranch string) bool {
 			switch repo {
-			case "go", "net", "sys":
-				return branch == "master" && goBranch == "master"
+			case "go":
+				return atLeastGo1(goBranch, 19)
+			case "net", "sys":
+				return branch == "master" && atLeastGo1(goBranch, 19)
 			default:
 				return false
 			}

dashboard/builders_test.go

@@ -406,7 +406,11 @@ func TestBuilderConfig(t *testing.T) {
 		{b("linux-amd64", "net"), both},
 
 		{b("linux-loong64-3a5000", "go"), onlyPost},
+		{b("linux-loong64-3a5000@go1.99", "go"), onlyPost},
+		{b("linux-loong64-3a5000@go1.18", "go"), none}, // Go 1.18 doesn't support this port.
 		{b("linux-loong64-3a5000", "sys"), onlyPost},
+		{b("linux-loong64-3a5000@go1.99", "sys"), onlyPost},
+		{b("linux-loong64-3a5000@go1.18", "sys"), none},
 		{b("linux-loong64-3a5000", "net"), onlyPost},
 
 		// OpenBSD 6.8.