[clang] [llvm] [RISCV] Improve casting between i1 scalable vectors and i8 fixed vectors for -mrvv-vector-bits (PR #139190)

[Paul Walker via cfe-commits]

https://github.com/paulwalker-arm approved this pull request.

This looks good to me. Thanks for helping to unblock https://github.com/llvm/llvm-project/pull/130973 @topperc.

https://github.com/llvm/llvm-project/pull/139190