Fix warnings (causing CI failures) with new Rust beta.

- Panic messages must now be string literals (we used `format!()` in
  many places; `panic!()` can take format strings directly).
- Some dead enum options with EVEX encoding stuff in old x86 backend.
  This will go away soon and/or be moved to the new backend anyway, so
  let's silence the warning for now.
- A few other misc warnings.

cranelift/frontend/src/ssa.rs

@@ -1098,7 +1098,7 @@ mod tests {
             Ok(()) => {}
             Err(_errors) => {
                 #[cfg(feature = "std")]
-                panic!(_errors);
+                panic!("{}", _errors);
                 #[cfg(not(feature = "std"))]
                 panic!("function failed to verify");
             }
@@ -1291,7 +1291,7 @@ mod tests {
             Ok(()) => {}
             Err(_errors) => {
                 #[cfg(feature = "std")]
-                panic!(_errors);
+                panic!("{}", _errors);
                 #[cfg(not(feature = "std"))]
                 panic!("function failed to verify");
             }
@@ -1357,7 +1357,7 @@ mod tests {
             Ok(()) => {}
             Err(_errors) => {
                 #[cfg(feature = "std")]
-                panic!(_errors);
+                panic!("{}", _errors);
                 #[cfg(not(feature = "std"))]
                 panic!("function failed to verify");
             }