[llvm] [RISCV] Model dest EEW and fix peepholes not checking EEW (PR #105945)

[Philip Reames via llvm-commits]

preames wrote:

As discussed over at (https://github.com/llvm/llvm-project/pull/104689#discussion_r1731379411), I think we really need to be checking EEW here not the ratio.  

Given that, I think we also have a problem with narrowing operations as MI.  (This case is not yet in the current code, but something I'd like to handle long term.)  Such operations have SrcEEW=SEW*2.  It would be nice if we could find a way to solve this consistently.  (i.e. Can we implement something like an getOperandEEW?)

https://github.com/llvm/llvm-project/pull/105945