[MLIR] RemoveDeadValues pass errors on unreachable operation

[Sirraide]

Consider (https://godbolt.org/z/7c7oqqEY7):

module @test {
  func.func @foo() {
    %false = arith.constant false
    %zero = arith.constant 0 : i64
    cf.cond_br %false, ^bb1, ^bb4
  ^bb1:
    %3 = arith.cmpi eq, %zero, %zero : i64
    cf.br ^bb1
  ^bb4:
    return
  }
}

For some reason --remove-dead-values chokes on this and errors rather than just removing the block

<source>:7:10: error: null operand found
    %3 = arith.cmpi eq, %zero, %zero : i64
         ^
<source>:7:10: note: see current operation: %1 = "arith.cmpi"(<<NULL VALUE>>, <<NULL VALUE>>) <{predicate = 0 : i64}> : (<<NULL TYPE>>, <<NULL TYPE>>) -> i1

Not sure if #153732 is in any way related but in both cases the cause of the problem seems to be an unreachable block.

[llvmbot]

@llvm/issue-subscribers-mlir

Author: None (Sirraide)

Consider (https://godbolt.org/z/7c7oqqEY7): ```mlir module @test { func.func @foo() { %false = arith.constant false %zero = arith.constant 0 : i64 cf.cond_br %false, ^bb1, ^bb4 ^bb1: %3 = arith.cmpi eq, %zero, %zero : i64 cf.br ^bb1 ^bb4: return } } ``` For some reason `--remove-dead-values` chokes on this and errors rather than just removing the block ```console <source>:7:10: error: null operand found %3 = arith.cmpi eq, %zero, %zero : i64 ^ <source>:7:10: note: see current operation: %1 = "arith.cmpi"(<<NULL VALUE>>, <<NULL VALUE>>) <{predicate = 0 : i64}> : (<<NULL TYPE>>, <<NULL TYPE>>) -> i1 ```

Not sure if #153732 is in any way related but in both cases the cause of the problem seems to be an unreachable block.