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

[Philip Reames via llvm-commits]

preames wrote:

In concept, sound reasonable.  I had played with a more generic vmerge operand swap a while back, and got lost in unprofitable cases.  Something restricted specifically to the extend doesn't seem unreasonable.  Would need a real patch and tests to meaningful review beyond that.  

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