Add some tests for constant propagation

The results aren't ideal but they represent the current state.

src/test/mir-opt/const_prop/array_index.rs

@@ -0,0 +1,33 @@
+fn main() {
+    let x: u32 = [0, 1, 2, 3][2];
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _2 = [const 0u32, const 1u32, const 2u32, const 3u32];
+//      ...
+//      _3 = const 2usize;
+//      _4 = const 4usize;
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb1;
+//  }
+//  bb1: {
+//      _1 = _2[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _5 = const true;
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb1;
+//  }
+//  bb1: {
+//      _1 = _2[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.after.mir

src/test/mir-opt/const_prop/checked_add.rs

@@ -0,0 +1,21 @@
+// compile-flags: -C overflow-checks=on
+
+fn main() {
+    let x: u32 = 1 + 1;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _2 = CheckedAdd(const 1u32, const 1u32);
+//      assert(!move (_2.1: bool), "attempt to add with overflow") -> bb1;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _2 = (const 2u32, const false);
+//      assert(!move (_2.1: bool), "attempt to add with overflow") -> bb1;
+//  }
+// END rustc.main.ConstProp.after.mir

src/test/mir-opt/const_prop/slice_len.rs

@@ -0,0 +1,37 @@
+fn test() -> &'static [u32] {
+    &[1, 2]
+}
+
+fn main() {
+    let x = test()[0];
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb1: {
+//      ...
+//      _3 = const 0usize;
+//      _4 = Len((*_2));
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb2;
+//  }
+//  bb2: {
+//      _1 = (*_2)[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _3 = const 0usize;
+//      _4 = Len((*_2));
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb2;
+//  }
+//  bb2: {
+//      _1 = (*_2)[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.after.mir