Separate miri/ctfe unsupported operations

Author: oli-obk

src/librustc_middle/mir/interpret/error.rs

@@ -451,6 +451,8 @@ pub enum UnsupportedOpInfo {
     NoMirFor(DefId),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
+
+    // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     /// Encountered raw bytes where we needed a pointer.
     ReadBytesAsPointer,
 }