Allow binding immediates to instructions (#1012)

This change should make the code more clear (and less code) when adding encodings for instructions with specific immediates; e.g., a constant with a 0 immediate could be encoded as an XOR with something like `const.bind(...)` without explicitly creating the necessary predicates. It has several parts:
* Introduce Bindable trait to instructions
* Convert all instruction bindings to use Bindable::bind()
* Add ability to bind immediates to BoundInstruction
This is an attempt to reduce some of the issues in #955.

cranelift/codegen/meta/src/shared/legalize.rs

@@ -1,5 +1,5 @@
 use crate::cdsl::ast::{var, ExprBuilder, Literal};
-use crate::cdsl::instructions::{Instruction, InstructionGroup};
+use crate::cdsl::instructions::{Bindable, Instruction, InstructionGroup};
 use crate::cdsl::xform::{TransformGroupBuilder, TransformGroups};
 
 use crate::shared::immediates::Immediates;