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)

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

@@ -27,8 +27,6 @@ import (
 func LoadPackage(filenames []string) {
 	base.Timer.Start("fe", "parse")
 
-	mode := syntax.CheckBranches
-
 	// Limit the number of simultaneously open files.
 	sem := make(chan struct{}, runtime.GOMAXPROCS(0)+10)
 
@@ -58,7 +56,7 @@ func LoadPackage(filenames []string) {
 				}
 				defer f.Close()
 
-				p.file, _ = syntax.Parse(fbase, f, p.error, p.pragma, mode) // errors are tracked via p.error
+				p.file, _ = syntax.Parse(fbase, f, p.error, p.pragma, syntax.CheckBranches) // errors are tracked via p.error
 			}()
 		}
 	}()

src/cmd/compile/internal/types2/api.go

@@ -128,9 +128,8 @@ type Config struct {
 	//          Do not use casually!
 	FakeImportC bool
 
-	// If IgnoreLabels is set, correct label use is not checked.
-	// TODO(gri) Consolidate label checking and remove this flag.
-	IgnoreLabels bool
+	// If IgnoreBranchErrors is set, branch/label errors are ignored.
+	IgnoreBranchErrors bool
 
 	// If CompilerErrorMessages is set, errors are reported using
 	// cmd/compile error strings to match $GOROOT/test errors.

src/cmd/compile/internal/types2/stmt.go

@@ -41,7 +41,7 @@ func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body
 
 	check.stmtList(0, body.List)
 
-	if check.hasLabel && !check.conf.IgnoreLabels {
+	if check.hasLabel && !check.conf.IgnoreBranchErrors {
 		check.labels(body)
 	}
 
@@ -504,23 +504,18 @@ func (check *Checker) stmt(ctxt stmtContext, s syntax.Stmt) {
 			check.hasLabel = true
 			break // checked in 2nd pass (check.labels)
 		}
+		if check.conf.IgnoreBranchErrors {
+			break
+		}
 		switch s.Tok {
 		case syntax.Break:
 			if ctxt&breakOk == 0 {
-				if check.conf.CompilerErrorMessages {
-					check.error(s, "break is not in a loop, switch, or select statement")
-				} else {
 				check.error(s, "break not in for, switch, or select statement")
 			}
-			}
 		case syntax.Continue:
 			if ctxt&continueOk == 0 {
-				if check.conf.CompilerErrorMessages {
-					check.error(s, "continue is not in a loop")
-				} else {
 				check.error(s, "continue not in for statement")
 			}
-			}
 		case syntax.Fallthrough:
 			if ctxt&fallthroughOk == 0 {
 				var msg string