Adds per-builtin instructions, removing `BuiltinInst`. This collapses
`builtin_inst_kind.def` into `inst_kind.def` so that we have a single
place for all macro uses. I still want to remove `BuiltinInstKind`, but
it's something I think is better separated from the `BuiltinInst`
removal.
I'm collapsing the build targets `ids` and `inst_kind` into one because
they both have links to builtin kind information now. It's hard to
separate without a cycle. I'm using the `typed_insts` name because that
seems like the actual most significant thing there, and more interesting
relative to the `inst` target.
Jon Ross-Perkins
josh11bandJosh L
