[llvm] [RISCV][TTI] Implement instruction cost for vp_merge. (PR #112327)

[Luke Lau via llvm-commits]

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lukel97 wrote:

Nit, but should we just move this test into `rvv-select.ll` if that's the logic that it's going to test? I don't think we have any separate cost model test files just for VP intrinsics

https://github.com/llvm/llvm-project/pull/112327