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/machinst/blockorder.rs

@@ -1,6 +1,7 @@
 //! Computation of basic block order in emitted code.
 
 use crate::machinst::*;
+use regalloc::{BlockIx, Function};
 
 /// Simple reverse postorder-based block order emission.
 ///
@@ -29,9 +30,8 @@ impl BlockRPO {
             }
         }
 
-        let (start, end) = &vcode.block_ranges[block as usize];
-        for i in *start..*end {
-            if vcode.insts[i as usize].is_epilogue_placeholder() {
+        for i in vcode.block_insns(BlockIx::new(block)) {
+            if vcode.get_insn(i).is_epilogue_placeholder() {
                 debug_assert!(self.deferred_last.is_none());
                 self.deferred_last = Some(block);
                 return;