ValueTypes.td: Reorganize ValueType

Introduce VTAny as `isOverloaded = true`. ValueType.isOverloaded` is used for;

  - Define `iPTRAny`, `vAny`, `fAny`, and `Any`
  - Reflect `ValueType.isOverloaded` to `LLVMType.isAny` in `Intrinsics.td`
  - (Planninig) Reflect the condition to `MVT::isOverloaded()`

Introduce some fields in `ValueType`

  - LLVMName
  - isInteger
  - isFP
  - isVector
  - isScalable
  - nElem
  - ElementType

Introduce `VTVec<int nelem, ValueType elt, int value>` to represent
the element type. VTVec.Size may be calculated by `!mul(nelem, elt.Size)`.

Introduce `list<ValueType> ValueTypes` as convention for lookup.

Differential Revision: https://reviews.llvm.org/D146179