Implement SSA-based reference propagation.

compiler/rustc_mir_transform/src/lib.rs

@@ -84,6 +84,7 @@ mod match_branches;
 mod multiple_return_terminators;
 mod normalize_array_len;
 mod nrvo;
+mod ref_prop;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
 mod remove_uninit_drops;
@@ -559,6 +560,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &separate_const_switch::SeparateConstSwitch,
             &simplify::SimplifyLocals::BeforeConstProp,
             &copy_prop::CopyProp,
+            &ref_prop::ReferencePropagation,
             &const_prop::ConstProp,
             &dataflow_const_prop::DataflowConstProp,
             //