This is PR refactors `alignedConversionPrecondition` from
VectorEmulateNarrowType.cpp and adds new helper hooks.
**Update `alignedConversionPrecondition` (1)**
This method doesn't require the vector type for the "container" argument. The
underlying element type is sufficient. The corresponding argument has been
renamed as `containerTy` - this is meant as the multi-byte container element
type (`i8`, `i16`, `i32`, etc). With this change, the updated invocations of
`alignedConversionPrecondition` (in e.g. `RewriteAlignedSubByteIntExt`) make it
clear that the container element type is assumed to be `i8`.
**Update alignedConversionPrecondition (2):**
The final check in `alignedConversionPrecondition` has been replaced with a new
helper method, `isSubByteVecFittable`. This helper hook is now also re-used in
`ConvertVectorTransferRead` (to improve code re-use).
**Other updates**
Extended + unified comments.
**Implements**: https://github.com/llvm/llvm-project/issues/123630
d928a671b8