cranelift-meta: Don't let-bind operand definitions (#5969)
We've adopted this pattern in Cranelift's instruction definitions where we let-bind some calls to `Operand::new` and then later use them in one or more calls to `Inst::new`. That pattern has two problems: - It puts the type of each operand somewhere potentially far removed from the instruction in which it's used. - We let-bind the same name for many different operands, compounding the first problem by making it harder to find _which_ definition is used. So instead this commit removes all let-bindings for operand definitions and constructs a new `Operand` every time. Constructing an `Operand` at every use means we duplicate some documentation strings, but not all that many of them as it turns out. I've left the let-bound type-sets alone, so those are currently still shared across many instructions. They have some of the same problems and should be reviewed as well.
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Jamey Sharp