[MHLO] Handle dynamic dimensions in HLO<->MHLO

[copybara-service]

[MHLO] Handle dynamic dimensions in HLO<->MHLO

• Fix creating constant zero for ConvertOp HLO->MHLO translation
• Fix broadcast in dim bounded lowering from MHLO->HLO
• Don't use StableHLO verification methods on MHLO ReshapeOp with bounded dynamic outputs

$ cat /tmp/t.hlo 
HloModule main, entry_computation_layout={(pred[<=1801,1]{1,0})->pred[<=1801,1]{1,0}}

ENTRY %convert_with_predicate (Arg_0.1: pred[<=1801,1]) -> pred[<=1801,1] {
  %Arg_0.1 = pred[<=1801,1] parameter(0)
  ROOT %convert_pred = pred[<=1801,1] convert(%Arg_0.1)
}

$ xla-translate /tmp/t.hlo --hlo-text-to-mlir-hlo
func.func @main(%arg0: tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>) -> tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>> {
  %0 = mhlo.constant dense<false> : tensor<1801x1xi1>
  %1 = "mhlo.get_dimension_size"(%arg0) <{dimension = 0 : i64}> : (tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>) -> tensor<i32>
  %2 = "mhlo.set_dimension_size"(%0, %1) <{dimension = 0 : i64}> : (tensor<1801x1xi1>, tensor<i32>) -> tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>
  %3 = mhlo.compare  NE, %arg0, %2 : (tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>, tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>) -> tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>
  return %3 : tensor<?x1xi1, #mhlo.type_extensions<bounds = [1801, ?]>>
}

Currently this fails when trying to create the mhlo.constant dense<false> that gets fed into compare since constants cannot have a bounded size.