compiler: annotate or fix fall through cases in switch statements

A new GCC warning, -Wimplicit-fallthrough, detected these three cases
    where a switch case falls through to another.  One of them looks like
    a bug, so I fixed it; the two other look like intended fall throughs
    so I just added a "fall through" comment to suppress the GCC warning.
    
    Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.
    
    Reviewed-on: https://go-review.googlesource.com/26695

From-SVN: r239332

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-8da2129a005cc1f44d4d993b0b7312b64c0d68a4
+5e05b7bc947231b4d5a8327bf63e2fa648e51dc7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/escape.cc

@@ -281,6 +281,7 @@ Node::op_format() const
 		{
 		case Runtime::PANIC:
 		  op << "panic";
+		  break;
 
 		case Runtime::APPEND:
 		  op << "append";
@@ -1923,6 +1924,7 @@ Escape_analysis_assign::assign(Node* dst, Node* src)
 	    if (!e->type()->has_pointer())
 	      break;
 	  }
+	  // Fall through.
 
 	case Expression::EXPRESSION_CONVERSION:
 	case Expression::EXPRESSION_TYPE_GUARD:

gcc/go/gofrontend/expressions.cc

@@ -5767,6 +5767,7 @@ Binary_expression::do_get_backend(Translate_context* context)
     case OPERATOR_DIV:
       if (left_type->float_type() != NULL || left_type->complex_type() != NULL)
         break;
+      // Fall through.
     case OPERATOR_MOD:
       is_idiv_op = true;
       break;