[llvm] [VPlan] Implement VPWidenCallRecipe::computeCost (NFCI). (PR #106047)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

Looks like this causes buildbot failures.

In my case, stage1-clang crashes while compiling ADTTests on the aarch64 host.

```
FAILED: unittests/ADT/CMakeFiles/ADTTests.dir/BitVectorTest.cpp.o 
clang++: llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7366: llvm::VectorizationFactor llvm::LoopVectorizationPlanner::computeBestVF(): Assertion `(BestFactor.Width == LegacyVF.Width || planContainsAdditionalSimplifications(getPlanFor(BestFactor.Width), CostCtx, OrigLoop)) && " VPlan cost model and legacy cost model disagreed"' failed.
```

https://github.com/llvm/llvm-project/pull/106047