Remove duplicated temporaries creating during box derefs elaboration

Temporaries created with `MirPatch::new_temp` will be declared after
patch application. Remove manually created duplicate declarations.

Removing duplicates exposes another issue. Visitor elaborates
terminator twice and attempts to access new, but not yet available,
local declarations. Remove duplicated call to `visit_terminator`.

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

@@ -70,7 +70,6 @@ impl<'tcx, 'a> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'tcx, 'a> {
                 build_ptr_tys(tcx, base_ty.boxed_ty(), self.unique_did, self.nonnull_did);
 
             let ptr_local = self.patch.new_temp(ptr_ty, source_info.span);
-            self.local_decls.push(LocalDecl::new(ptr_ty, source_info.span));
 
             self.patch.add_statement(location, StatementKind::StorageLive(ptr_local));
 
@@ -125,13 +124,6 @@ impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs {
                     index += 1;
                 }
 
-                if let Some(terminator) = terminator
-                && !matches!(terminator.kind, TerminatorKind::Yield{..})
-                {
-                    let location = Location { block, statement_index: index };
-                    visitor.visit_terminator(terminator, location);
-                }
-
                 let location = Location { block, statement_index: index };
                 match terminator {
                     // yielding into a box is handled when lowering generators