[llvm] [InlineCost] Consider the default branch when calculating cost (PR #77856)

[Craig Topper via llvm-commits]

topperc wrote:

Someone just reported to me internally that this caused a benchmark regression in our downstream.

I wonder if the default case was originally missing because it should just be a fallthrough after all the other cases are picked off. It shouldn't have any code. Given N destinations for a switch you only need N-1 compares and branches. Maybe an unreachable default should reduce the number of case clusters since one case cluster check wouldn't be needed? Though I'm not sure exactly what the backend generates.

https://github.com/llvm/llvm-project/pull/77856