[llvm] [FunctionSpecializer] Keep the blocks in dead functions and remove the callsites of dead function properly. (PR #154668)

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XChy wrote:

> (not immediately clear to me why, `i32 undef` when querying the known constant), thus `%unreachable_call` gets executable again.

Is it because the specialized callee will not return?

https://github.com/llvm/llvm-project/pull/154668