[meta] Introduce the InstructionGroupBuilder;

This follows the rest of the code base data structures, where we have a
mutable data structure builder; once the data structure is constructed,
it's immutable.

This also makes the definition of instructions easier, and it paves the
way for defining immediate variants.

cranelift/codegen/meta/src/cdsl/xform.rs

@@ -404,11 +404,14 @@ impl TransformGroups {
 #[should_panic]
 fn test_double_custom_legalization() {
     use crate::cdsl::formats::{FormatRegistry, InstructionFormatBuilder};
-    use crate::cdsl::instructions::InstructionBuilder;
+    use crate::cdsl::instructions::{InstructionBuilder, InstructionGroupBuilder};
 
     let mut format = FormatRegistry::new();
     format.insert(InstructionFormatBuilder::new("nullary"));
-    let dummy_inst = InstructionBuilder::new("dummy", "doc").finish(&format);
+    let mut inst_group = InstructionGroupBuilder::new("test", "", &format);
+    inst_group.push(InstructionBuilder::new("dummy", "doc"));
+    let inst_group = inst_group.finish();
+    let dummy_inst = inst_group.by_name("dummy");
 
     let mut transform_group = TransformGroupBuilder::new("test", "doc");
     transform_group.custom_legalize(&dummy_inst, "custom 1");