fix(qbe): use extraData instead of child3 for hIf else-branch

Block lowering overwrites child3 for statement chaining.
Else block is stored in extraData by hir_lower (matches c_backend).
Fixes QBE parse error 'last block misses jump' in if-without-else chains.

src_bux/parser.bux

@@ -298,6 +298,10 @@ func parserParsePrimary(p: *Parser) -> *Expr {
 }
 
 // ---------------------------------------------------------------------------
+
+func dummyFunc() -> int {
+    return 0;
+}
 // Postfix: call, index, field access, as, is, ?
 // ---------------------------------------------------------------------------