[Liftoff][cleanup] Fix comment and use register constant

The comment in {CallRuntime} was misleading. The {rsi}/{esi} register
did not hold the instance, but the context instead. The generated code
was correct thought.

R=titzer@chromium.org

Change-Id: I18020a04ac75faedf7ad5e4b07cab27ae0aedae7
Reviewed-on: https://chromium-review.googlesource.com/1051232Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53088}

src/wasm/baseline/ia32/liftoff-assembler-ia32.h

@@ -1557,8 +1557,8 @@ void LiftoffAssembler::CallNativeWasmCode(Address addr) {
 }
 
 void LiftoffAssembler::CallRuntime(Zone* zone, Runtime::FunctionId fid) {
-  // Set instance to zero.
-  xor_(esi, esi);
+  // Set context to zero (Smi::kZero) for the runtime call.
+  xor_(kContextRegister, kContextRegister);
   CallRuntimeDelayed(zone, fid);
 }
 

src/wasm/baseline/x64/liftoff-assembler-x64.h

@@ -1410,8 +1410,8 @@ void LiftoffAssembler::CallNativeWasmCode(Address addr) {
 }
 
 void LiftoffAssembler::CallRuntime(Zone* zone, Runtime::FunctionId fid) {
-  // Set instance to zero.
-  xorp(rsi, rsi);
+  // Set context to zero (Smi::kZero) for the runtime call.
+  xorp(kContextRegister, kContextRegister);
   CallRuntimeDelayed(zone, fid);
 }