[llvm] [CostModel] Remove some negative costs. (PR #135533)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. Looks like those RISC-V insertvalues get lowered to COPYs which may or may not end up as vmv.v.vs, or otherwise a register rename.

https://github.com/llvm/llvm-project/pull/135533