fix: emit pre-range statements to outer scope in ForRange

[xgopilot]

Requested by @xushiwei

Fixes goplus/xgo#2629

Summary

• When ForRange() enters a new block scope, any statements emitted during range expression compilation (e.g., auto-generated type assertions like _autoGo_1, _ := doc.(map[string]any)) were incorrectly placed in the for-loop body instead of the outer scope
• RangeAssignThen now extracts these pre-range statements using clearBlockStmt()
• End emits them to the outer scope before the for-range statement

Changes

• stmt.go: Added preStmts field to forRangeStmt, extract pre-range statements in RangeAssignThen, emit them in End

Test plan

• All existing gogen tests pass (go test ./...)
• The XGo test case cl/_testgop/dql7 in goplus/xgo will validate this fix end-to-end

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.94%. Comparing base (2210ad3) to head (b174b81).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #593   +/-   ##
=======================================
  Coverage   95.94%   95.94%           
=======================================
  Files          22       22           
  Lines        6850     6853    +3     
=======================================
+ Hits         6572     6575    +3     
  Misses        207      207           
  Partials       71       71           

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[xushiwei]

@xgopilot Add a test case for this scenario

[xgopilot]

Test case added successfully!

I've added TestForRangePreStmts to xgo_test.go (line 1088-1127) which validates the fix for goplus/xgo#2629.

What the test does

The test creates a scenario where a type assertion statement is emitted during range expression compilation:

_autoGo_1, _ := data.(map[string]interface{})
for k, v := range _autoGo_1 {
    fmt.Println(k, v)
}

The key behavior being tested:

• When ForRange() enters a new block scope, any statements emitted during range expression compilation (like the type assertion above) should be extracted by RangeAssignThen() using clearBlockStmt()
• These pre-statements should be emitted to the outer scope before the for-range statement, not inside the loop body

Test results

✅ All tests pass (go test ./...)

The test has been committed and pushed to the PR branch.