[liftoff] Prefer kWasmInstanceRegister for instance cache

The wasm instance will initially be in kWasmInstanceRegister, and for
each call we also need to put it in that register. Hence, when getting a
new register to cache the instance, prefer that register, if it is
available.

R=thibaudm@chromium.org

Bug: v8:11336
Change-Id: Ie7026c4c7c5e4b825b9ab310839f0273bd3ce7f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2743885
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73303}

src/wasm/baseline/liftoff-assembler.h

@@ -271,11 +271,17 @@ class LiftoffAssembler : public TurboAssembler {
 
     Register TrySetCachedInstanceRegister(LiftoffRegList pinned) {
       DCHECK_EQ(no_reg, cached_instance);
-      LiftoffRegList candidates = kGpCacheRegList.MaskOut(pinned);
-      if (!has_unused_register(candidates)) return no_reg;
-      SetInstanceCacheRegister(unused_register(candidates).gp());
-      DCHECK_NE(no_reg, cached_instance);
-      return cached_instance;
+      LiftoffRegList available_regs =
+          kGpCacheRegList.MaskOut(pinned).MaskOut(used_registers);
+      if (available_regs.is_empty()) return no_reg;
+      // Prefer the {kWasmInstanceRegister}, because that's where the instance
+      // initially is, and where it needs to be for calls.
+      Register new_cache_reg = available_regs.has(kWasmInstanceRegister)
+                                   ? kWasmInstanceRegister
+                                   : available_regs.GetFirstRegSet().gp();
+      SetInstanceCacheRegister(new_cache_reg);
+      DCHECK_EQ(new_cache_reg, cached_instance);
+      return new_cache_reg;
     }
 
     void ClearCachedInstanceRegister() {