[wasm] [decoder] Handle degenerate br_table correctly

The degenerate br_table case should be handled specially only in the
graph building consumer. There it is necessary for avoiding the
construction of a degenerate Switch node, which would cause a DCHECK
error in instruction selection.
For other backends, like the baseline compiler, we should handle it as
a br_table, because the signature is different to a br.

Drive-by: Fix redundant validation.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ia430b6d251eb1323848977388ed95a112f8c76f7
Reviewed-on: https://chromium-review.googlesource.com/715616Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48494}

src/wasm/function-body-decoder-impl.h

@@ -1474,19 +1474,8 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             }
             if (!VALIDATE(this->ok())) break;
 
-            if (operand.table_count > 0) {
-              interface_.BrTable(this, operand, key);
-            } else {
-              // Only a default target. Do the equivalent of br.
-              BranchTableIterator<validate> iterator(this, operand);
-              const byte* pos = iterator.pc();
-              uint32_t target = iterator.next();
-              if (!VALIDATE(target < control_.size())) {
-                this->error(pos, "improper branch in br_table");
-                break;
-              }
-              interface_.BreakTo(this, target);
-            }
+            interface_.BrTable(this, operand, key);
+
             len = 1 + iterator.length();
             EndControl();
             break;

src/wasm/function-body-decoder.cc

@@ -314,6 +314,13 @@ class WasmGraphBuildingInterface {
 
   void BrTable(Decoder* decoder, const BranchTableOperand<true>& operand,
                const Value& key) {
+    if (operand.table_count == 0) {
+      // Only a default target. Do the equivalent of br.
+      uint32_t target = BranchTableIterator<true>(decoder, operand).next();
+      BreakTo(decoder, target);
+      return;
+    }
+
     SsaEnv* break_env = ssa_env_;
     // Build branches to the various blocks based on the table.
     TFNode* sw = BUILD(Switch, operand.table_count + 1, key.node);