Add a comment about drop(start_bx)

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -258,6 +258,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    // The builders will be created separately for each basic block at `codegen_block`.
+    // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
     // Codegen the body of each block using reverse postorder