[Liftoff][arm64] Use wzr/xzr for pushing zero constants

This is an optimization to avoid an unneeded "mov <reg>, #0"
instruction. Instead, we can just directly use the zero register.

R=ahaas@chromium.org

Bug: chromium:854011, v8:6600
Change-Id: I187d7a659c42d7d4a6d5798eddff8b7ee0983bbc
Reviewed-on: https://chromium-review.googlesource.com/1124684
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54186}

src/wasm/baseline/arm64/liftoff-assembler-arm64.h

@@ -957,27 +957,19 @@ void LiftoffStackSlots::Construct() {
         asm_->Poke(liftoff::GetRegFromType(slot.src_.reg(), slot.src_.type()),
                    poke_offset);
         break;
-      case LiftoffAssembler::VarState::KIntConst: {
-        switch (slot.src_.type()) {
-          case kWasmI32: {
-            UseScratchRegisterScope temps(asm_);
-            Register scratch = temps.AcquireW();
-            asm_->Mov(scratch, slot.src_.i32_const());
-            asm_->Poke(scratch, poke_offset);
-            break;
-          }
-          case kWasmI64: {
-            UseScratchRegisterScope temps(asm_);
-            Register scratch = temps.AcquireX();
-            asm_->Mov(scratch, int64_t{slot.src_.i32_const()});
-            asm_->Poke(scratch, poke_offset);
-            break;
-          }
-          default:
-            UNREACHABLE();
+      case LiftoffAssembler::VarState::KIntConst:
+        DCHECK(slot.src_.type() == kWasmI32 || slot.src_.type() == kWasmI64);
+        if (slot.src_.i32_const() == 0) {
+          Register zero_reg = slot.src_.type() == kWasmI32 ? wzr : xzr;
+          asm_->Poke(zero_reg, poke_offset);
+        } else {
+          UseScratchRegisterScope temps(asm_);
+          Register scratch = slot.src_.type() == kWasmI32 ? temps.AcquireW()
+                                                          : temps.AcquireX();
+          asm_->Mov(scratch, int64_t{slot.src_.i32_const()});
+          asm_->Poke(scratch, poke_offset);
         }
         break;
-      }
     }
     slot_index++;
   }