[FXML-5083] SCFToEmitC: use already lowered operands

[cferry-AMD]

The lowering of scf.for ops uses the non-lowered operands, which breaks when these operands have undergone EmitC lowering.

[cferry-AMD]

NB: upstream is also incorrect, so I'll issue the same upstream.

[mgehre-amd]

That was quick!
Could you please add a test using index?

[cferry-AMD]

On the upstream maybe; on the current version of our fork, something like this:

func.func @for_yield_index(%arg0 : index, %arg1 : index, %arg2 : index) -> index {
  %zero = arith.constant 0 : index
  %r = scf.for %i0 = %arg0 to %arg1 step %arg2 iter_args(%acc = %zero) -> index {
    scf.yield %acc : index
  }
  return %r : index
}

results in a bug:

mlir/test/Conversion/SCFToEmitC/for.mlir:105:5: error: null operand found
    scf.yield %acc : index
    ^
mlir/test/Conversion/SCFToEmitC/for.mlir:105:5: note: see current operation: "emitc.assign"(%5, <<NULL VALUE>>) : (!emitc.size_t, <<NULL TYPE>>) -> ()

(the scf.yield is indeed erased, so it should not be verified, yet it is).