[wasm] Fix decoder tracing

This fixes two issues with --trace-wasm-decoder and --trace-liftoff.
First, cache state tracing in liftoff only makes sense together with
--trace-wasm-decoder, so disable it otherwise.
Second, we were sometimes hitting a DCHECK with --trace-wasm-decoder on
functions which bailed out to Turbofan, since the bailout was resetting
the end pointer of the decoder to the current pc.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ib4e052a1cdec6389b12b4a0ea57f07c031324ed5
Reviewed-on: https://chromium-review.googlesource.com/853493
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50415}

src/wasm/baseline/liftoff-compiler.cc

@@ -771,7 +771,7 @@ class LiftoffCompiler {
 
   void TraceCacheState(Decoder* decoder) const {
 #ifdef DEBUG
-    if (!FLAG_trace_liftoff) return;
+    if (!FLAG_trace_liftoff || !FLAG_trace_wasm_decoder) return;
     OFStream os(stdout);
     for (int control_depth = decoder->control_depth() - 1; control_depth >= -1;
          --control_depth) {

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

@@ -1871,22 +1871,25 @@ class WasmFullDecoder : public WasmDecoder<validate> {
           TRACE_PART(" %c@%d:%s", WasmOpcodes::ShortNameOf(val.type),
                      static_cast<int>(val.pc - this->start_),
                      WasmOpcodes::OpcodeName(opcode));
+          // If the decoder failed, don't try to decode the operands, as this
+          // can trigger a DCHECK failure.
+          if (this->failed()) continue;
           switch (opcode) {
             case kExprI32Const: {
-              ImmI32Operand<validate> operand(this, val.pc);
+              ImmI32Operand<Decoder::kNoValidate> operand(this, val.pc);
               TRACE_PART("[%d]", operand.value);
               break;
             }
             case kExprGetLocal:
             case kExprSetLocal:
             case kExprTeeLocal: {
-              LocalIndexOperand<Decoder::kValidate> operand(this, val.pc);
+              LocalIndexOperand<Decoder::kNoValidate> operand(this, val.pc);
               TRACE_PART("[%u]", operand.index);
               break;
             }
             case kExprGetGlobal:
             case kExprSetGlobal: {
-              GlobalIndexOperand<validate> operand(this, val.pc);
+              GlobalIndexOperand<Decoder::kNoValidate> operand(this, val.pc);
               TRACE_PART("[%u]", operand.index);
               break;
             }