[Liftoff] Two small fixes

They are uncovered by existing test cases once we support globals and
memory operations.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I66c7143b66c816ab9a032c18bf6b2c82f7291f68
Reviewed-on: https://chromium-review.googlesource.com/756705Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49174}

src/wasm/baseline/liftoff-assembler.cc

@@ -147,6 +147,7 @@ void LiftoffAssembler::CacheState::InitMerge(const CacheState& source,
     } else {
       // Keep this a stack slot (which is the initial value).
       DCHECK(slot->is_stack());
+      ++slot;
       return;
     }
     *slot = VarState(reg);

src/wasm/baseline/liftoff-assembler.h

@@ -191,7 +191,7 @@ class LiftoffAssembler : public TurboAssembler {
   }
 
   uint32_t GetNumUses(Register reg) const {
-    DCHECK_GT(CacheState::kMaxRegisterCode, reg.code());
+    DCHECK_GE(CacheState::kMaxRegisterCode, reg.code());
     return cache_state_.register_use_count[reg.code()];
   }