[llvm] [RISCV] Optimize two source deinterleave2 via ri.vunzip2{a, b} (PR #142667)

[Philip Reames via llvm-commits]

preames wrote:

> Are we intentionally missing coverage for vunzip2b?

Not other than the fact it didn't seem interesting given the opcode and matching bits weren't changing.  Happy to duplicate the tests if you like?

https://github.com/llvm/llvm-project/pull/142667