Should `simplify-cfg` be aware of address taken blocks?

[vaivaswatha]

In #168, we added LLVM dialect support for block addresses. Block addresses are modeled as a llvm.blocktag operation in the block whose address is taken, and the actual address obtained from llvm.blockaddress that refers to the tag.

The verifier requirement is that llvm.blockaddress must always have a corresponding llvm.blocktag. If the block containing this llvm.blocktag is not reachable, simplify-cfg may eliminate it, thus invalidating the verifier requirement.

Should we have an interface for conveying that certain blocks cannot be removed even if unreachable? MLIR doesn't seem to care.

// mlir-opt -canonicalize --mlir-print-ir-after-all addr_taken_block.mlir

module {
  llvm.func @test_blockaddress() -> !llvm.ptr {
    // 1. Get the address of the block tagged with id = 42
    %addr = llvm.blockaddress <function = @test_blockaddress, tag = <id = 42>> : !llvm.ptr

    // 2. Normal control flow exits the function immediately
    llvm.return %addr : !llvm.ptr

  // 3. This block is isolated from standard CFG successors/predecessors
  ^indirect_target:
    llvm.blocktag <id = 42>
    llvm.return %addr : !llvm.ptr
  }
}

Running mlir-opt as commented above will result in the optimizer eliminating ^indirect_target, leading to a verifier failure later.