LibWasm: Fix validation of if-else blocks

We were doing a number of things wrong:
- Switching to the parent context in the else meant that we couldn't
  break out of the else section anymore
- We were not validating the resulting values, and so the stack was
  in a relatively unknown state after 'else'

This commit fixes these issues :^)

Userland/Libraries/LibWasm/AbstractMachine/Validator.h

@@ -260,7 +260,6 @@ private:
 
     enum class ChildScopeKind {
         Block,
-        Loop,
         IfWithoutElse,
         IfWithElse,
         Else,
@@ -270,7 +269,6 @@ private:
         size_t initial_stack_size { 0 };
         struct IfDetails {
             Stack initial_stack;
-            Stack true_branch_stack;
         };
         Variant<IfDetails, Empty> details;
     };