- 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).
The main enhancement in this commit is support for reference types and
stackmaps. This requires tracking whether each VReg is a "reference" or
"pointer". At certain instructions designated as "safepoints", the
regalloc will (i) ensure that all references are in spillslots rather
than in registers, and (ii) provide a list of exactly which spillslots
have live references at that program point. This can be used by, e.g., a
GC to trace and possibly modify pointers. The stackmap of spillslots is
precise: it includes all live references, and *only* live references.
This commit also brings in some API tweaks as part of the in-progress
Cranelift glue. In particular, it makes Allocations and Operands
mutually disjoint by using the same bitfield for the type-tag in both
and choosing non-overlapping tags. This will allow instructions to carry
an Operand for each register slot and then overwrite these in place with
Allocations. The `OperandOrAllocation` type does the necessary magic to
make this look like an enum, but staying in 32 bits.
