[llvm] [RISCV][VPlan] Cost vp.merges that are likely to be folded away as free (PR #209387)

[via llvm-commits]

github-actions[bot] wrote:

test-suite diff from f60650c772485640b57520e91cec9779cc7f27dc...697960b21791e412acb5789699e044f6486680e6: https://github.com/llvm/llvm-project/actions/runs/29312785559/artifacts/8304840247

Statistics: https://github.com/llvm/llvm-project/actions/runs/29312785559/artifacts/8304840779

## riscv64-linux-gnu `-march=rva23u64 -O3`
| ('Program', '')                             |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:--------------------------------------------|-------------------------:|-------------------------:|-------------------------:|
| test-suite...marks/7zip/7zip-benchmark.test |                   634014 |                   634148 |              0.000211352 |
| Geomean difference                          |                      nan |                      nan |              6.58347e-07 |
## aarch64-linux-gnu `-march=armv9-a -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |                        0 |
## x86_64-linux-gnu `-march=x86-64-v3 -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |                        0 |


https://github.com/llvm/llvm-project/pull/209387