remove type_use

This is broken, and results in poor performance due to the undefined
behaviour in the LLVM IR. LLVM's `mergefunc` is a *much* better way of
doing this since it merges based on the equality of the bytecode.

For example, consider `std::repr`. It generates different code per
type, but is not included in the type bounds of generics.

The `mergefunc` pass works for most of our code but currently hits an
assert on libstd. It is receiving attention upstream so it will be
ready soon, but I don't think removing this broken code should wait any
longer. I've opened #9536 about enabling it by default.

Closes #8651
Closes #3547
Closes #2537
Closes #6971
Closes #9222