Rollup merge of rust-lang#108739 - 823984418:patch-1, r=cjgillot

Prevent the `start_bx` basic block in codegen from having two `Builder`s at the same time

Here, at the same time, there are two `start_llbb` builder, this should be unexpected.

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -258,6 +258,10 @@ 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
  for (bb, _) in traversal::reverse_postorder(&mir) {
  fx.codegen_block(bb);