wasm: Remove duplicated parameters when popping an If

Parameters are duplicated when pushing an If block, so they're available
to the Else block without an extra heap allocation. However, when
truncating the stack after popping the If control frame, the stack size
at entry doesn't account for the duplicated parameters. That is
intentional: the Else block uses this value to know what's the stack
size when it is entered, so there's nothing to change there.

This patch makes the wasm translation truncates the value stack to the
right size after an If block, by taking those duplicated parameters into
account.

cranelift/wasm/src/code_translator.rs

@@ -323,13 +323,33 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                 // since we truncate the stack back to the original height
                 // below.
             }
+
             builder.switch_to_block(next_block);
             builder.seal_block(next_block);
+
             // If it is a loop we also have to seal the body loop block
             if let ControlStackFrame::Loop { header, .. } = frame {
                 builder.seal_block(header)
             }
-            state.stack.truncate(frame.original_stack_size());
+
+            // The "If" frame pushes its parameters twice, so they're available to the else block
+            // (see also `FuncTranslationState::push_if`).
+            // Yet, the original_stack_size member accounts for them only once, so that the else
+            // block can see the same number of parameters as the consequent block. As a matter of
+            // fact, we need to substract an extra number of parameter values for if blocks.
+            let num_duplicated_params = if let ControlStackFrame::If {
+                num_param_values, ..
+            } = frame
+            {
+                debug_assert!(num_param_values <= frame.original_stack_size());
+                num_param_values
+            } else {
+                0
+            };
+
+            state
+                .stack
+                .truncate(frame.original_stack_size() - num_duplicated_params);
             state
                 .stack
                 .extend_from_slice(builder.block_params(next_block));