Add more regression tests

Author: oli-obk

src/test/ui/consts/const-prop-ice2.rs

@@ -0,0 +1,5 @@
+fn main() {
+    enum Enum { One=1 }
+    let xs=[0;1 as usize];
+    println!("{}", xs[Enum::One as usize]); //~ ERROR the len is 1 but the index is 1
+}

src/test/ui/consts/const-prop-ice2.stderr

@@ -0,0 +1,10 @@
+error: index out of bounds: the len is 1 but the index is 1
+  --> $DIR/const-prop-ice2.rs:4:20
+   |
+LL |     println!("{}", xs[Enum::One as usize]); //~ ERROR the len is 1 but the index is 1
+   |                    ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: #[deny(const_err)] on by default
+
+error: aborting due to previous error
+