[llvm] [SimplifyCFG] Relax `cttz` cost check in `simplifySwitchOfPowersOfTwo` (PR #145159)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

Only relaxing cost for now, need to take a better look on permitting reachable default cases too (currently miscompiles https://alive2.llvm.org/ce/z/wG8GoT).

https://github.com/llvm/llvm-project/pull/145159