pub fn allocatable_registers(
has_feature: impl FnMut(&str) -> bool,
) -> FxHashMap<InlineAsmRegClass, FxHashSet<InlineAsmReg>>
Returns the full set of allocatable registers for a given architecture.
The registers are structured as a map containing the set of allocatable registers in each register class. A particular register may be allocatable from multiple register classes, in which case it will appear multiple times in the map.