[llvm] [RISCV] Preserve tail agnostic policy in some vector peepholes (PR #105788)

[Luke Lau via llvm-commits]

lukel97 wrote:

Rebased to add an LLVM test case showing the codegen diff for the vmerge.vvm -> vmv.v.v peephole

https://github.com/llvm/llvm-project/pull/105788