[mlir] scf::ForOp: Drop iter arguments (and corresponding result) with no use

'ForOpIterArgsFolder' can now remove iterator arguments (and corresponding
results) with no use.

Example:

```
%cst = constant 32 : i32

%0:2 = scf.for %arg1 = %lb to %ub step %step iter_args(%arg2 = %arg0, %arg3 = %cst)
  -> (i32, i32) {
  %1 = addu %arg2, %cst : i32
  scf.yield %1, %1 : i32, i32
}

use(%0#0)

```

%arg3 is not used in the block, and its corresponding result `%0#1` has no use,
thus remove the iter argument.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D98711