[llvm] [RISCV] Avoid VMNOT by swapping VMERGE operands for mask extensions (PR #126751)

[Piotr Fusik via llvm-commits]

pfusik wrote:

> LGTM
> 
> Please make sure to update the review description to be non-blank before landing.

Done.
Also added negative tests for XOR that is not a NOT. Force-pushed because of editing the first commit with the tests.

> Do you have commit access? Or do you need me to land for you?

I do. Since the GitHub interface is restricted to squash merges, I'll cherry-pick the commit adding the tests to the main branch. I've done that before.

https://github.com/llvm/llvm-project/pull/126751