[WIP] rustc_mir_transform: Add extra ConstProp pass to fold constants at end of MIR-opt

compiler/rustc_mir_transform/src/lib.rs

@@ -490,6 +490,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &o1(simplify::SimplifyCfg::new("final")),
             &nrvo::RenameReturnPlace,
             &const_debuginfo::ConstDebugInfo,
+            &const_prop::ConstProp,
             &simplify::SimplifyLocals,
             &multiple_return_terminators::MultipleReturnTerminators,
             &deduplicate_blocks::DeduplicateBlocks,

src/test/mir-opt/const_folding_test.f.PreCodegen.after.mir

@@ -0,0 +1,10 @@
+// MIR for `f` after PreCodegen
+
+fn f() -> usize {
+    let mut _0: usize;                   // return place in scope 0 at $DIR/const_folding_test.rs:3:11: 3:16
+
+    bb0: {
+        _0 = const 2_usize;              // scope 0 at $DIR/const_folding_test.rs:4:5: 4:29
+        return;                          // scope 0 at $DIR/const_folding_test.rs:5:2: 5:2
+    }
+}

src/test/mir-opt/const_folding_test.rs

@@ -0,0 +1,9 @@
+// EMIT_MIR const_folding_test.f.PreCodegen.after.mir
+
+fn f() -> usize {
+    1 + if true {1} else {2}
+}
+
+fn main() {
+    let _a = f();
+}