[llvm] [CGP] Do not eliminate blocks which have their address taken (PR #163962)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> This sounds like a band aid to not get the undefined symbol error, if we spot the missing location to call UpdateForDeletedBlock. But I have the feeling the code is broken afterwards. We are just placing the symbol somewhere.

blockaddress constants are specifically designed to be used as inputs to indirectbr.  Any other use of a blockaddress constant is outside of the IR model.  In particular, if a block has no indirectbr predecessors, we provide no guarantees at all about the value of a blockaddress.  So... in cases like #161164, yes, it just needs to point somewhere.

https://github.com/llvm/llvm-project/pull/163962