Better pack fields in Variable

Used a BitField to for Variable fields instead of relying on the compiler, saving some memory probably.
This reduces sizeof(Variable) from 64 to 40 on x64

BUG=v8:5209

Committed: https://crrev.com/955606506c256ea389d6c4a8e07babfea512d190
Review-Url: https://codereview.chromium.org/2257493002
Cr-Original-Commit-Position: refs/heads/master@{#38891}
Cr-Commit-Position: refs/heads/master@{#38967}

src/ast/variables.h

@@ -49,6 +49,7 @@ class Variable final : public ZoneObject {
     return ForceContextAllocationField::decode(bit_field_);
   }
   void ForceContextAllocation() {
+    DCHECK(IsUnallocated() || IsContextSlot());
     bit_field_ = ForceContextAllocationField::update(bit_field_, true);
   }
   bool is_used() { return IsUsedField::decode(bit_field_); }