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/s390x/inst/imms.rs

@@ -56,11 +56,6 @@ impl SImm20 {
         }
     }
 
-    /// Create a zero immediate of this format.
-    pub fn zero() -> SImm20 {
-        SImm20 { value: 0 }
-    }
-
     /// Bits for encoding.
     pub fn bits(&self) -> u32 {
         let encoded: u32 = self.value as u32;
@@ -130,11 +125,6 @@ impl UImm16Shifted {
             shift: self.shift,
         }
     }
-
-    /// Returns the value that this constant represents.
-    pub fn value(&self) -> u64 {
-        (self.bits as u64) << (16 * self.shift)
-    }
 }
 
 /// A 32-bit immediate with a {0,32}-bit shift.
@@ -179,31 +169,12 @@ impl UImm32Shifted {
         }
     }
 
-    pub fn from_uimm16shifted(value: UImm16Shifted) -> UImm32Shifted {
-        if value.shift % 2 == 0 {
-            UImm32Shifted {
-                bits: value.bits as u32,
-                shift: value.shift / 2,
-            }
-        } else {
-            UImm32Shifted {
-                bits: (value.bits as u32) << 16,
-                shift: value.shift / 2,
-            }
-        }
-    }
-
     pub fn negate_bits(&self) -> UImm32Shifted {
         UImm32Shifted {
             bits: !self.bits,
             shift: self.shift,
         }
     }
-
-    /// Returns the value that this constant represents.
-    pub fn value(&self) -> u64 {
-        (self.bits as u64) << (32 * self.shift)
-    }
 }
 
 impl PrettyPrint for UImm12 {