'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
0a74a7161b