[wasm] Remove negative implication from --single-threaded to --liftoff

The negative implication from --single-threaded to --liftoff was
introduced because with --single-threaded, no tier-up to TurboFan is
possible, and TurboFan provides faster code and smaller code

This CL removes this implication. The reason is that this implication
does not only define the default compiler for --single-threaded to
TurboFan, and also completely disables the option to use Liftoff
instead. By removing the implication, and embedder who uses
--single-threaded can decide by themselves if they want to use Liftoff
by setting --liftoff, or if they don't want to use Liftoff by setting
--no-liftoff.

R=clemensb@chromium.org

Bug: v8:11279
Change-Id: Ie6e64a42823b87484135364ecb4589cfd188db5e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2599548Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71917}

src/flags/flag-definitions.h

@@ -823,10 +823,8 @@ DEFINE_NEG_IMPLICATION(fuzzing, liftoff_only)
 DEFINE_BOOL(experimental_liftoff_extern_ref, false,
             "enable support for externref in Liftoff")
 // We can't tier up (from Liftoff to TurboFan) in single-threaded mode, hence
-// disable Liftoff in that configuration for now. The alternative is disabling
-// TurboFan, which would reduce peak performance considerably.
-// Note that for debugging, Liftoff will still be used.
-DEFINE_NEG_IMPLICATION(single_threaded, liftoff)
+// disable tier up in that configuration for now.
+DEFINE_NEG_IMPLICATION(single_threaded, wasm_tier_up)
 DEFINE_DEBUG_BOOL(trace_liftoff, false,
                   "trace Liftoff, the baseline compiler for WebAssembly")
 DEFINE_BOOL(trace_wasm_memory, false,

tools/testrunner/local/variants.py

@@ -55,7 +55,7 @@ INCOMPATIBLE_FLAGS_PER_VARIANT = {
   "assert_types": ["--no-assert-types"],
   "jitless": ["--opt", "--always-opt", "--liftoff", "--track-field-types", "--validate-asm"],
   "no_wasm_traps": ["--wasm-trap-handler"],
-  "nooptimization": ["--opt", "--always-opt", "--no-liftoff", "--predictable", "--wasm-tier-up"],
+  "nooptimization": ["--opt", "--always-opt", "--no-liftoff", "--wasm-tier-up"],
   "slow_path": ["--no-force-slow-path"],
   "stress_concurrent_allocation": ["--single-threaded-gc", "--predictable"],
   "stress_incremental_marking": ["--no-stress-incremental-marking"],