Cranelift: Make Opcode represented as a u8 instead of u16 and remove vestigial conversion impls (#5042)

* Cranelift: Make `Opcode` represented as a `u8` instead of `u16`

* Cranelift: Remove unused conversion impls for `Opcode`

These are vestigial, left over from Peepmatic.

cranelift/codegen/meta/src/gen_inst.rs

@@ -405,7 +405,7 @@ fn gen_opcodes(all_inst: &AllInstructions, fmt: &mut Formatter) {
         All instructions from all supported ISAs are present.
     "#,
     );
-    fmt.line("#[repr(u16)]");
+    fmt.line("#[repr(u8)]");
     fmt.line("#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]");
     fmt.line(
         r#"#[cfg_attr(
@@ -572,24 +572,6 @@ fn gen_opcodes(all_inst: &AllInstructions, fmt: &mut Formatter) {
     fmt.empty_line();
 }
 
-fn gen_try_from(all_inst: &AllInstructions, fmt: &mut Formatter) {
-    fmt.line("impl core::convert::TryFrom<u16> for Opcode {");
-    fmt.indent(|fmt| {
-        fmt.line("type Error = ();");
-        fmt.line("#[inline]");
-        fmt.line("fn try_from(x: u16) -> Result<Self, ()> {");
-        fmt.indent(|fmt| {
-            fmtln!(fmt, "if 0 < x && x <= {} {{", all_inst.len());
-            fmt.indent(|fmt| fmt.line("Ok(unsafe { core::mem::transmute(x) })"));
-            fmt.line("} else {");
-            fmt.indent(|fmt| fmt.line("Err(())"));
-            fmt.line("}");
-        });
-        fmt.line("}");
-    });
-    fmt.line("}");
-}
-
 /// Get the value type constraint for an SSA value operand, where
 /// `ctrl_typevar` is the controlling type variable.
 ///
@@ -1420,8 +1402,6 @@ pub(crate) fn generate(
     gen_opcodes(all_inst, &mut fmt);
     fmt.empty_line();
     gen_type_constraints(all_inst, &mut fmt);
-    fmt.empty_line();
-    gen_try_from(all_inst, &mut fmt);
     fmt.update_file(opcode_filename, out_dir)?;
 
     // ISLE DSL.

cranelift/codegen/src/ir/instructions.rs

@@ -7,9 +7,7 @@
 //! directory.
 
 use alloc::vec::Vec;
-use core::convert::{TryFrom, TryInto};
 use core::fmt::{self, Display, Formatter};
-use core::num::NonZeroU32;
 use core::ops::{Deref, DerefMut};
 use core::str::FromStr;
 
@@ -77,24 +75,6 @@ impl Opcode {
     }
 }
 
-impl TryFrom<NonZeroU32> for Opcode {
-    type Error = ();
-
-    #[inline]
-    fn try_from(x: NonZeroU32) -> Result<Self, ()> {
-        let x: u16 = x.get().try_into().map_err(|_| ())?;
-        Self::try_from(x)
-    }
-}
-
-impl From<Opcode> for NonZeroU32 {
-    #[inline]
-    fn from(op: Opcode) -> NonZeroU32 {
-        let x = op as u8;
-        NonZeroU32::new(x as u32).unwrap()
-    }
-}
-
 // This trait really belongs in cranelift-reader where it is used by the `.clif` file parser, but since
 // it critically depends on the `opcode_name()` function which is needed here anyway, it lives in
 // this module. This also saves us from running the build script twice to generate code for the two