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/mod.rs

@@ -1,8 +1,5 @@
 //! This module defines aarch64-specific machine instruction types.
 
-// Some variants are not constructed, but we still want them as options in the future.
-#![allow(dead_code)]
-
 use crate::binemit::{Addend, CodeOffset, Reloc};
 use crate::ir::types::{
     B1, B128, B16, B32, B64, B8, F32, F64, FFLAGS, I128, I16, I32, I64, I8, I8X16, IFLAGS, R32, R64,
@@ -2738,9 +2735,11 @@ pub enum LabelUse {
     /// 26-bit branch offset (unconditional branches). PC-rel, offset is imm << 2. Immediate is 26
     /// signed bits, in bits 25:0. Used by b, bl.
     Branch26,
+    #[allow(dead_code)]
     /// 19-bit offset for LDR (load literal). PC-rel, offset is imm << 2. Immediate is 19 signed bits,
     /// in bits 23:5.
     Ldr19,
+    #[allow(dead_code)]
     /// 21-bit offset for ADR (get address of label). PC-rel, offset is not shifted. Immediate is
     /// 21 signed bits, with high 19 bits in bits 23:5 and low 2 bits in bits 30:29.
     Adr21,