Add TargetIsa::as_any for downcasting to specific ISA implementations

This is necessary when we would like to check specific ISA flags, e.g.

cranelift/codegen/src/machinst/adapter.rs

@@ -10,6 +10,7 @@ use crate::settings::Flags;
 #[cfg(feature = "testing_hooks")]
 use crate::regalloc::RegDiversions;
 
+use core::any::Any;
 use std::borrow::Cow;
 use std::fmt;
 use target_lexicon::Triple;
@@ -127,4 +128,8 @@ impl TargetIsa for TargetIsaAdapter {
     fn unsigned_sub_overflow_condition(&self) -> ir::condcodes::IntCC {
         self.backend.unsigned_sub_overflow_condition()
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }