[const-prop] Fix ICE when casting function pointers

This fixes an ICE when building libcore with `-Z mir-opt-level=3`.

src/librustc_mir/transform/const_prop.rs

@@ -525,12 +525,10 @@ fn replace_with_const(
         source_info: SourceInfo,
     ) {
         trace!("attepting to replace {:?} with {:?}", rval, value);
-        self.ecx.validate_operand(
-            value,
-            vec![],
-            None,
-            true,
-        ).expect("value should already be a valid const");
+        if let Err(e) = self.ecx.validate_operand(value, vec![], None, true) {
+            trace!("validation error, attempt failed: {:?}", e);
+            return;
+        }
 
         // FIXME> figure out what tho do when try_read_immediate fails
         let imm = self.use_ecx(source_info, |this| {

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

+fn main() {
+    let _ = main as usize as *const fn();
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _3 = const main as fn() (Pointer(ReifyFnPointer));
+//      _2 = move _3 as usize (Misc);
+//      ...
+//      _1 = move _2 as *const fn() (Misc);
+//      ...
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _3 = const Scalar(AllocId(1).0x0) : fn();
+//      _2 = move _3 as usize (Misc);
+//      ...
+//      _1 = const Scalar(AllocId(1).0x0) : *const fn();
+//      ...
+//  }
+// END rustc.main.ConstProp.after.mir