Replace legacy ConstProp by GVN.

Author: cjgillot

compiler/rustc_mir_transform/src/const_prop.rs

@@ -553,7 +553,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &separate_const_switch::SeparateConstSwitch,
             &gvn::GVN,
             &simplify::SimplifyLocals::AfterGVN,
-            &const_prop::ConstProp,
             &dataflow_const_prop::DataflowConstProp,
             //
             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.