[wasm] Fix {WasmDecoder::OpcodeLength} for rethrow.

R=clemensh@chromium.org
TEST=unittests/WasmOpcodeLengthTest.Statements
BUG=v8:8091

Change-Id: Ia2a01d5c35ace33c9c8a29a8df7b30e3f1e119cc
Reviewed-on: https://chromium-review.googlesource.com/c/1278728Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56606}

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

@@ -1009,6 +1009,7 @@ class WasmDecoder : public Decoder {
         MemoryAccessImmediate<validate> imm(decoder, pc, UINT32_MAX);
         return 1 + imm.length;
       }
+      case kExprRethrow:
       case kExprBr:
       case kExprBrIf: {
         BreakDepthImmediate<validate> imm(decoder, pc);

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -2818,6 +2818,7 @@ TEST_F(WasmOpcodeLengthTest, Statements) {
   EXPECT_LENGTH(2, kExprBr);
   EXPECT_LENGTH(2, kExprBrIf);
   EXPECT_LENGTH(2, kExprThrow);
+  EXPECT_LENGTH(2, kExprRethrow);
   EXPECT_LENGTH(2, kExprCatch);
   EXPECT_LENGTH_N(2, kExprBlock, kLocalI32);
   EXPECT_LENGTH_N(2, kExprLoop, kLocalI32);