[llvm] IR: Simplify BlockAddress replacement (PR #135360)

[Nikita Popov via llvm-commits]

nikic wrote:

@arsenm See https://llvm.org/docs/LangRef.html#addresses-of-basic-blocks:

> This value only has defined behavior when used as an operand to the ‘[indirectbr](https://llvm.org/docs/LangRef.html#i-indirectbr)’ or for comparisons against null. Pointer equality tests between labels addresses results in undefined behavior — though, again, comparison against null is ok, and no label is equal to the null pointer. This may be passed around as an opaque pointer sized value as long as the bits are not inspected. This allows ptrtoint and arithmetic to be performed on these values so long as the original value is reconstituted before the indirectbr instruction.

If a block is deleted, we still must guarantee that the corresponding blockaddress compares as non-null.

https://github.com/llvm/llvm-project/pull/135360