[meta] Remove name lookups in formats;

This does a lot at once, since there was no clear way to split the three
commits:

- Instruction need to be passed an explicit InstructionFormat,
- InstructionFormat deduplication is checked once all entities have been
defined;

cranelift/codegen/meta/src/isa/arm64/mod.rs

@@ -49,11 +49,7 @@ pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let settings = define_settings(&shared_defs.settings);
     let regs = define_registers();
 
-    let inst_group = InstructionGroupBuilder::new(
-        &mut shared_defs.all_instructions,
-        &shared_defs.format_registry,
-    )
-    .build();
+    let inst_group = InstructionGroupBuilder::new(&mut shared_defs.all_instructions).build();
 
     let mut a64 = CpuMode::new("A64");