[llvm] [IR] Don't store switch case values as operands (PR #170984)

[Alexis Engelke via llvm-commits]

aengelke wrote:

Compared to previous version: Fixes in SimplifyCFG and Instruction::isIdenticalToWhenDefined. I local stage2 build now passes tests.

https://github.com/llvm/llvm-project/pull/170984