[llvm] [RISCV] Lower VP_SELECT constant false to use vmerge.vxm/vmerge.vim (PR #144461)

[Liao Chunyu via llvm-commits]

ChunyuLiao wrote:

> Does #146695 also fix the issue in the godbolt example?

It didn't solve the issue, and without https://github.com/llvm/llvm-project/pull/146695 could generate vp_select, but after this patch, it generates select.

https://github.com/llvm/llvm-project/pull/144461