* `optimize_mode` is passed to `link.File` and stored there
* improve the debugging function `Module.dumpInst`
* get rid of `Value.the_one_possible_value` in favor of a few more
specific values for different types. This is less buggy, one less
footgun.
* `Type.onePossibleValue` now returns a `?Value` instead of `bool`.
* codegen handles undefined values. `undef` is a new `MCValue` tag.
It uses 0xaa values depending on optimization mode. However
optimization mode does not yet support scope overrides.
* link.zig: move the `Options` field from `File.Elf` and `File.C` to
the base struct.
- fix the Tag enum to adhere to style conventions
* ZIR now supports emitting undefined values.
* Fix the logic of comptime math to properly compare against zero using
the `compareWithZero` function.