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/postopt.rs

@@ -364,19 +364,17 @@ pub fn do_postopt(func: &mut Function, isa: &dyn TargetIsa) {
     while let Some(_block) = pos.next_block() {
         let mut last_flags_clobber = None;
         while let Some(inst) = pos.next_inst() {
-            if isa.uses_cpu_flags() {
+            if !is_mach_backend && isa.uses_cpu_flags() {
                 // Optimize instructions to make use of flags.
                 optimize_cpu_flags(&mut pos, inst, last_flags_clobber, isa);
 
-                if !is_mach_backend {
-                    // Track the most recent seen instruction that clobbers the flags.
-                    if let Some(constraints) = isa
-                        .encoding_info()
-                        .operand_constraints(pos.func.encodings[inst])
-                    {
-                        if constraints.clobbers_flags {
-                            last_flags_clobber = Some(inst)
-                        }
+                // Track the most recent seen instruction that clobbers the flags.
+                if let Some(constraints) = isa
+                    .encoding_info()
+                    .operand_constraints(pos.func.encodings[inst])
+                {
+                    if constraints.clobbers_flags {
+                        last_flags_clobber = Some(inst)
                     }
                 }
             }