ir: use value.Value for Scope of InstCatchPad and InstCleanupPad

Note: this commit is targetted for the v0.4 release.

Updates llir#50.

Author: mewmew

ir/inst_other.go

@@ -558,7 +558,8 @@ type InstCatchPad struct {
 	// Name of local variable associated with the result.
 	LocalIdent
 	// Exception scope.
-	Scope *TermCatchSwitch // TODO: rename to From? rename to Within?
+	// TODO: rename to From? rename to Within?
+	Scope value.Value // *ir.TermCatchSwitch
 	// Exception arguments.
 	//
 	// Arg has one of the following underlying types:
@@ -616,7 +617,8 @@ type InstCleanupPad struct {
 	// Name of local variable associated with the result.
 	LocalIdent
 	// Exception scope.
-	Scope ExceptionScope // TODO: rename to Parent? rename to From?
+	// TODO: rename to Parent? rename to From?
+	Scope value.Value // ExceptionScope
 	// Exception arguments.
 	//
 	// Arg has one of the following underlying types: