[wasm] OpcodeLength should detect invalid opcodes

OpcodeLength in function-body-decoder was implemented in a way that did
not detect invalid non-prefixed opcodes, even when {validate} was on.
This CL brings its behavior in line with prefixed opcodes and validation
requirements.

Change-Id: I53fec32f13bd18a2ed0c7a7666d69fc09603db56
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2552516
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71368}

test/unittests/wasm/loop-assignment-analysis-unittest.cc

@@ -183,6 +183,12 @@ TEST_F(WasmLoopAssignmentAnalyzerTest, Malformed) {
   CHECK_NULL(assigned);
 }
 
+TEST_F(WasmLoopAssignmentAnalyzerTest, InvalidOpcode) {
+  byte code[] = {WASM_LOOP(0xFF)};
+  BitVector* assigned = Analyze(code, code + arraysize(code));
+  EXPECT_EQ(assigned, nullptr);
+}
+
 TEST_F(WasmLoopAssignmentAnalyzerTest, regress_642867) {
   static const byte code[] = {
       WASM_LOOP(WASM_ZERO, kExprLocalSet, 0xFA, 0xFF, 0xFF, 0xFF,