[dev.ssa] cmd/compile/internal/gc: handle _ label correctly

An empty label statement can just be ignored, as it cannot
be the target of any gotos.

Tests are already in test/fixedbugs/issue7538*.go

Fixes #11589
Fixes #11593

Change-Id: Iadcd639e7200ce16aa40fd7fa3eaf82522513e82
Reviewed-on: https://go-review.googlesource.com/12093
Reviewed-by: Daniel Morsing <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: randall77

src/cmd/compile/internal/gc/ssa.go

@@ -316,6 +316,11 @@ func (s *state) stmt(n *Node) {
 		s.assign(OAS, n.Left.Name.Heapaddr, palloc)
 
 	case OLABEL, OGOTO:
+		if n.Op == OLABEL && isblanksym(n.Left.Sym) {
+			// Empty identifier is valid but useless.
+			// See issues 11589, 11593.
+			return
+		}
 		// get block at label, or make one
 		t := s.labels[n.Left.Sym.Name]
 		if t == nil {