Restore CodeGen/LowLevelType from `Support`

This is rework of;
  - D30046 (LLT)

Since I have introduced `llvm-min-tblgen` as D146352, `llvm-tblgen`
may depend on `CodeGen`.

`LowLevlType.h` originally belonged to `CodeGen`. Almost all userse are
still under `CodeGen` or `Target`. I think `CodeGen` is the right place
to put `LowLevelType.h`.

`MachineValueType.h` may be moved as well. (later, D149024)

I have made many modules depend on `CodeGen`. It is consistent but
inefficient. It will be split out later, D148769

Besides, I had to isolate MVT and LLT in modmap, since
`llvm::PredicateInfo` clashes between `TableGen/CodeGenSchedule.h`
and `Transforms/Utils/PredicateInfo.h`.
(I think better to introduce namespace llvm::TableGen)

Depends on D145937, D146352, and D148768.

Differential Revision: https://reviews.llvm.org/D148767