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/num_uses.rs

@@ -1,15 +1,9 @@
 //! A pass that computes the number of uses of any given instruction.
 
-#![allow(dead_code)]
-#![allow(unused_imports)]
-
-use crate::cursor::{Cursor, FuncCursor};
-use crate::dce::has_side_effect;
 use crate::entity::SecondaryMap;
 use crate::ir::dfg::ValueDef;
-use crate::ir::instructions::InstructionData;
 use crate::ir::Value;
-use crate::ir::{DataFlowGraph, Function, Inst, Opcode};
+use crate::ir::{DataFlowGraph, Function, Inst};
 
 /// Auxiliary data structure that counts the number of uses of any given
 /// instruction in a Function. This is used during instruction selection
@@ -51,16 +45,6 @@ impl NumUses {
         }
     }
 
-    /// How many times is an instruction used?
-    pub fn use_count(&self, i: Inst) -> usize {
-        self.uses[i] as usize
-    }
-
-    /// Is an instruction used at all?
-    pub fn is_used(&self, i: Inst) -> bool {
-        self.use_count(i) > 0
-    }
-
     /// Take the complete uses map, consuming this analysis result.
     pub fn take_uses(self) -> SecondaryMap<Inst, u32> {
         self.uses