cmd/compile: do branch/label checks only once

The previous change implemented the missing fallthrough checking
in the parser. Therefore we can now disable the duplicate check
in the type checker:

- rename (types2.Config.)IngoreLabels to IgnoreBranches to more
  accurately reflect its functionality

- now also ignore break/continue/fallthroughs, not just labels

The IgnoreBranches flag only exists for types2, for use with
the compiler. There's no need to port this code to go/types.

Note: An alternative (and perhaps better) approach would be
to not use the the parser's CheckBranches mode and instead
enable (i.e. not disable) the branch/label checking in the
type checker. However, this requires a bit more work because
the type checker's error messages about goto's jumping over
variables don't have access to the variable names, which are
desired in the error messages.

Fixes #51456.

Change-Id: Ib2e71e811d4e84e4895b729646e879fd43b12dcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/414135
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/noder/irgen.go

@@ -41,7 +41,7 @@ func checkFiles(noders []*noder) (posMap, *types2.Package, *types2.Info) {
 	conf := types2.Config{
 		Context:               ctxt,
 		GoVersion:             base.Flag.Lang,
-		IgnoreLabels:          true, // parser already checked via syntax.CheckBranches mode
+		IgnoreBranchErrors:    true, // parser already checked via syntax.CheckBranches mode
 		CompilerErrorMessages: true, // use error strings matching existing compiler errors
 		Error: func(err error) {
 			terr := err.(types2.Error)