This patch is a subpart of D125768 intented to make the review easier.
The `SizedOp` struct represents operations to be performed on a certain number of bytes.
It is responsible for breaking them down into platform types and forwarded to the `Backend`.
The `Backend` struct represents a lower level abstraction that works only on types (`uint8_t`, `__m128i`, ...).
It is similar to instruction selection.
Differential Revision: https://reviews.llvm.org/D126768
67fe3bd33c