Do not remove unused definitions inside GVN.

compiler/rustc_mir_transform/src/lib.rs

@@ -570,6 +570,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &separate_const_switch::SeparateConstSwitch,
             &const_prop::ConstProp,
             &gvn::GVN,
+            &simplify::SimplifyLocals::AfterGVN,
             &dataflow_const_prop::DataflowConstProp,
             &const_debuginfo::ConstDebugInfo,
             &o1(simplify_branches::SimplifyConstCondition::AfterConstProp),