[Liftoff][cleanup] Use more Register instead of LiftoffRegister

We generally try to use the most specific type, but sometimes we still
used LiftoffRegister even though it's statically known that it always
holds a GP register. Make these uses use Register directly. Note that
the conversion between Register and LiftoffRegister is a noop in
Release builds.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I1cf9aa727fb6fd71bbc1eed77df5e9d01e35ddee
Reviewed-on: https://chromium-review.googlesource.com/c/1278727Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56605}

src/wasm/baseline/liftoff-assembler.cc

@@ -449,7 +449,7 @@ void LiftoffAssembler::SpillAllRegisters() {
 void LiftoffAssembler::PrepareCall(FunctionSig* sig,
                                    compiler::CallDescriptor* call_descriptor,
                                    Register* target,
-                                   LiftoffRegister* target_instance) {
+                                   Register* target_instance) {
   uint32_t num_params = static_cast<uint32_t>(sig->parameter_count());
   // Input 0 is the call target.
   constexpr size_t kInputShift = 1;
@@ -472,10 +472,12 @@ void LiftoffAssembler::PrepareCall(FunctionSig* sig,
   compiler::LinkageLocation instance_loc =
       call_descriptor->GetInputLocation(kInputShift);
   DCHECK(instance_loc.IsRegister() && !instance_loc.IsAnyRegister());
-  LiftoffRegister instance_reg(Register::from_code(instance_loc.AsRegister()));
+  Register instance_reg = Register::from_code(instance_loc.AsRegister());
   param_regs.set(instance_reg);
   if (target_instance && *target_instance != instance_reg) {
-    stack_transfers.MoveRegister(instance_reg, *target_instance, kWasmIntPtr);
+    stack_transfers.MoveRegister(LiftoffRegister(instance_reg),
+                                 LiftoffRegister(*target_instance),
+                                 kWasmIntPtr);
   }
 
   // Now move all parameter values into the right slot for the call.
@@ -551,7 +553,7 @@ void LiftoffAssembler::PrepareCall(FunctionSig* sig,
 
   // Reload the instance from the stack.
   if (!target_instance) {
-    FillInstanceInto(instance_reg.gp());
+    FillInstanceInto(instance_reg);
   }
 }
 

src/wasm/baseline/liftoff-assembler.h

@@ -318,7 +318,7 @@ class LiftoffAssembler : public TurboAssembler {
   // register, or {no_reg} if target was spilled to the stack.
   void PrepareCall(FunctionSig*, compiler::CallDescriptor*,
                    Register* target = nullptr,
-                   LiftoffRegister* target_instance = nullptr);
+                   Register* target_instance = nullptr);
   // Process return values of the call.
   void FinishCall(FunctionSig*, compiler::CallDescriptor*);