[wasm] Fix compilation tiers for TurboFan-only

If --no-liftoff is passed, we should never compile with Liftoff,
independent of the --wasm-tier-up flag.
The current logic hard-coded Liftoff as baseline tier and TurboFan as
top tier for the tiering case.

Drive-by: Remove unused {default_tier_} field from
          {CompilationUnitBuilder}.

R=ahaas@chromium.org

Bug: chromium:1040061
Change-Id: I65bc5e4aad85396e3b952b41cfe1150f6cd29890
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1992439Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65662}

src/wasm/function-compiler.cc

@@ -110,7 +110,7 @@ std::unique_ptr<WasmInstructionBuffer> WasmInstructionBuffer::New(size_t size) {
 // End of PIMPL interface WasmInstructionBuffer for WasmInstBufferImpl
 
 // static
-ExecutionTier WasmCompilationUnit::GetDefaultExecutionTier(
+ExecutionTier WasmCompilationUnit::GetBaselineExecutionTier(
     const WasmModule* module) {
   // Liftoff does not support the special asm.js opcodes, thus always compile
   // asm.js modules with TurboFan.

src/wasm/function-compiler.h

@@ -76,7 +76,7 @@ struct WasmCompilationResult {
 
 class V8_EXPORT_PRIVATE WasmCompilationUnit final {
  public:
-  static ExecutionTier GetDefaultExecutionTier(const WasmModule*);
+  static ExecutionTier GetBaselineExecutionTier(const WasmModule*);
 
   WasmCompilationUnit(int index, ExecutionTier tier)
       : func_index_(index), tier_(tier) {}

src/wasm/module-compiler.cc

@@ -668,17 +668,15 @@ ExecutionTierPair GetRequestedExecutionTiers(
     const WasmFeatures& enabled_features, uint32_t func_index) {
   ExecutionTierPair result;
 
+  result.baseline_tier = WasmCompilationUnit::GetBaselineExecutionTier(module);
   switch (compile_mode) {
     case CompileMode::kRegular:
-      result.baseline_tier =
-          WasmCompilationUnit::GetDefaultExecutionTier(module);
       result.top_tier = result.baseline_tier;
       return result;
 
     case CompileMode::kTiering:
 
       // Default tiering behaviour.
-      result.baseline_tier = ExecutionTier::kLiftoff;
       result.top_tier = ExecutionTier::kTurbofan;
 
       // Check if compilation hints override default tiering behaviour.
@@ -711,9 +709,7 @@ ExecutionTierPair GetRequestedExecutionTiers(
 class CompilationUnitBuilder {
  public:
   explicit CompilationUnitBuilder(NativeModule* native_module)
-      : native_module_(native_module),
-        default_tier_(WasmCompilationUnit::GetDefaultExecutionTier(
-            native_module->module())) {}
+      : native_module_(native_module) {}
 
   void AddUnits(uint32_t func_index) {
     if (func_index < native_module_->module()->num_imported_functions) {
@@ -775,7 +771,6 @@ class CompilationUnitBuilder {
   }
 
   NativeModule* const native_module_;
-  const ExecutionTier default_tier_;
   std::vector<WasmCompilationUnit> baseline_units_;
   std::vector<WasmCompilationUnit> tiering_units_;
   std::vector<std::shared_ptr<JSToWasmWrapperCompilationUnit>>