Address review comments:

- Undo temporary changes to default features (`all-arch`) and a
  signal-handler test.
- Remove `SIGTRAP` handler: no longer needed now that we've found an
  "undefined opcode" option on ARM64.
- Rename pp.rs to pretty_print.rs in machinst/.
- Only use empty stack-probe on non-x86. As per a comment in
  rust-lang/compiler-builtins [1], LLVM only supports stack probes on
  x86 and x86-64. Thus, on any other CPU architecture, we cannot refer
  to `__rust_probestack`, because it does not exist.
- Rename arm64 to aarch64.
- Use `target` directive in vcode filetests.
- Run the flags verifier, but without encinfo, when using new backends.
- Clean up warning overrides.
- Fix up use of casts: use u32::from(x) and siblings when possible,
  u32::try_from(x).unwrap() when not, to avoid silent truncation.
- Take immutable `Function` borrows as input; we don't actually
  mutate the input IR.
- Lots of other miscellaneous cleanups.

[1] cae3e6ea23/src/probestack.rs (L39)

cranelift/codegen/src/verifier/flags.rs

@@ -28,17 +28,18 @@ pub fn verify_flags(
     errors: &mut VerifierErrors,
 ) -> VerifierStepResult<()> {
     let _tt = timing::verify_flags();
-    if isa.is_none() || isa.unwrap().get_mach_backend().is_none() {
-        let mut verifier = FlagsVerifier {
-            func,
-            cfg,
-            encinfo: isa.map(|isa| isa.encoding_info()),
-            livein: SecondaryMap::new(),
-        };
-        verifier.check(errors)
+    let encinfo = if isa.is_none() || isa.unwrap().get_mach_backend().is_some() {
+        None
     } else {
-        Ok(())
-    }
+        Some(isa.unwrap().encoding_info())
+    };
+    let mut verifier = FlagsVerifier {
+        func,
+        cfg,
+        encinfo,
+        livein: SecondaryMap::new(),
+    };
+    verifier.check(errors)
 }
 
 struct FlagsVerifier<'a> {