- Support preferred and non-preferred subsets of a register class. This
allows allocating, e.g., caller-saved registers before callee-saved
registers.
- Allow branch blockparam args to start an a certain offset in branch
operands; this allows branches to have other operands too (e.g.,
conditional-branch inputs).
- Allow `OperandOrAllocation` to be constructed from an `Allocation` and
`OperandKind` as well (i.e., an allocation with an use/def bit).
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