Narrow allow(dead_code) declarations (#4116)

* Narrow `allow(dead_code)` declarations

Having module wide `allow(dead_code)` may hide some code that's really
dead. In this commit I just narrowed the declarations to the specific
enum variants that were not used (as it seems reasonable to keep them
and their handling in all the matches, for future use). And the compiler
found more dead code that I think we can remove safely in the short
term.

With this, the only files annotated with a module-wide
`allow(dead_code)` are isle-generated files.

* resurrect some functions as test helpers

cranelift/codegen/src/isa/aarch64/inst/imms.rs

@@ -88,11 +88,6 @@ impl SImm7Scaled {
         }
     }
 
-    /// Create a zero immediate of this format.
-    pub fn zero(scale_ty: Type) -> SImm7Scaled {
-        SImm7Scaled { value: 0, scale_ty }
-    }
-
     /// Bits for encoding.
     pub fn bits(&self) -> u32 {
         let ty_bytes: i16 = self.scale_ty.bytes() as i16;
@@ -202,11 +197,6 @@ impl SImm9 {
         }
     }
 
-    /// Create a zero immediate of this format.
-    pub fn zero() -> SImm9 {
-        SImm9 { value: 0 }
-    }
-
     /// Bits for encoding.
     pub fn bits(&self) -> u32 {
         (self.value as u32) & 0x1ff
@@ -548,37 +538,6 @@ impl ImmLogic {
         // For every ImmLogical immediate, the inverse can also be encoded.
         Self::maybe_from_u64(!self.value, self.size.to_ty()).unwrap()
     }
-
-    /// This provides a safe(ish) way to avoid the costs of `maybe_from_u64` when we want to
-    /// encode a constant that we know at compiler-build time.  It constructs an `ImmLogic` from
-    /// the fields `n`, `r`, `s` and `size`, but in a debug build, checks that `value_to_check`
-    /// corresponds to those four fields.  The intention is that, in a non-debug build, this
-    /// reduces to something small enough that it will be a candidate for inlining.
-    pub fn from_n_r_s(value_to_check: u64, n: bool, r: u8, s: u8, size: OperandSize) -> Self {
-        // Construct it from the components we got given.
-        let imml = Self {
-            value: value_to_check,
-            n,
-            r,
-            s,
-            size,
-        };
-
-        // In debug mode, check that `n`/`r`/`s` are correct, given `value` and `size`.
-        debug_assert!(match ImmLogic::maybe_from_u64(
-            value_to_check,
-            if size == OperandSize::Size64 {
-                I64
-            } else {
-                I32
-            }
-        ) {
-            None => false, // fail: `value` is unrepresentable
-            Some(imml_check) => imml_check == imml,
-        });
-
-        imml
-    }
 }
 
 /// An immediate for shift instructions.
@@ -659,11 +618,6 @@ impl MoveWideConst {
             })
         }
     }
-
-    /// Returns the value that this constant represents.
-    pub fn value(&self) -> u64 {
-        (self.bits as u64) << (16 * self.shift)
-    }
 }
 
 /// Advanced SIMD modified immediate as used by MOVI/MVNI.