[InlineCost] Correct the default branch cost for the switch statement (#85160)

Fixes #81723.

The earliest commit of the related code is:
https://github.com/llvm/llvm-project/commit/919f9e8d65ada6552b8b8a5ec12ea49db91c922a.
I tried to understand the following code with
https://github.com/llvm/llvm-project/pull/77856#issuecomment-1993499085.

https://github.com/llvm/llvm-project/blob/5932fcc47855fdd209784f38820422d2369b84b2/llvm/lib/Analysis/InlineCost.cpp#L709-L720

I think only scenarios where there is a default branch were considered.