Reland "[test] Add Liftoff variant"

This is a reland of 712fa675.

Original change's description:
> [test] Add Liftoff variant
>
> Add a variant for testing the current state of the Liftoff
> implementation.
> This variant will only run on a subset of the bots, just like the
> --future variant.
>
> R=machenbach@chromium.org, hablich@chromium.org
>
> Bug: v8:7088, v8:6600
> Change-Id: If49fad3a8ed579356504b821a787326754f24e78
> Reviewed-on: https://chromium-review.googlesource.com/779420
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49504}

TBR=machenbach@chromium.org
CC=hablich@chromium.org

Bug: v8:7088, v8:6600
Change-Id: Ieb20020f07c70acaa64bb421763a41aa163a261b
Reviewed-on: https://chromium-review.googlesource.com/781499
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49531}

test/mjsunit/mjsunit.status

@@ -726,4 +726,11 @@
   'wasm/liftoff': [SKIP],
 }], # arch != x64 and arch != ia32
 
+##############################################################################
+['variant == liftoff', {
+  # In the liftoff variant, liftoff compilation happens even though the test
+  # does not explicitly enable it.
+  'wasm/default-liftoff-setting': [SKIP],
+}], # variant == liftoff
+
 ]

test/mjsunit/wasm/default-liftoff-setting.js

@@ -6,14 +6,16 @@
 
 // This test makes sure that by default, we do not compile with liftoff.
 
-load("test/mjsunit/wasm/wasm-constants.js");
-load("test/mjsunit/wasm/wasm-module-builder.js");
+load('test/mjsunit/wasm/wasm-constants.js');
+load('test/mjsunit/wasm/wasm-module-builder.js');
 
 const builder = new WasmModuleBuilder();
-builder.addFunction("i32_add", kSig_i_ii)
+builder.addFunction('i32_add', kSig_i_ii)
     .addBody([kExprGetLocal, 0, kExprGetLocal, 1, kExprI32Add])
     .exportFunc();
 
 const instance = builder.instantiate();
 
-assertFalse(%IsLiftoffFunction(instance.exports.i32_add));
+assertFalse(
+    %IsLiftoffFunction(instance.exports.i32_add),
+    'liftoff compilation should be off by default');

tools/testrunner/local/variants.py

@@ -6,6 +6,7 @@
 ALL_VARIANT_FLAGS = {
   "default": [[]],
   "future": [["--future"]],
+  "liftoff": [["--liftoff"]],
   "stress": [["--stress-opt", "--always-opt"]],
   # TODO(6792): Write protected code has been temporary added to the below
   # variant until the feature has been enabled (or staged) by default.
@@ -22,6 +23,7 @@ ALL_VARIANT_FLAGS = {
 FAST_VARIANT_FLAGS = {
   "default": [[]],
   "future": [["--future"]],
+  "liftoff": [["--liftoff"]],
   "stress": [["--stress-opt"]],
   # TODO(6792): Write protected code has been temporary added to the below
   # variant until the feature has been enabled (or staged) by default.
@@ -34,5 +36,6 @@ FAST_VARIANT_FLAGS = {
   "wasm_traps": [["--wasm_trap_handler", "--invoke-weak-callbacks"]],
 }
 
-ALL_VARIANTS = set(["default", "future", "stress", "stress_incremental_marking",
-                    "nooptimization", "stress_background_compile", "wasm_traps"])
+ALL_VARIANTS = set(["default", "future", "liftoff", "stress",
+                    "stress_incremental_marking", "nooptimization",
+                    "stress_background_compile", "wasm_traps"])

tools/testrunner/standard_runner.py

@@ -48,7 +48,7 @@ VARIANT_ALIASES = {
   # Additional variants, run on all bots.
   "more": MORE_VARIANTS,
   # Additional variants, run on a subset of bots.
-  "extra": ["future"],
+  "extra": ["future", "liftoff"],
 }
 
 GC_STRESS_FLAGS = ["--gc-interval=500", "--stress-compaction",