Support decrementing indices in DSLX for loop

DSLX loops are constrained to incrementing indices. For loop traversal, the developer may need to reverse their array prior and after the loop statement.

Example:
```
Array = Reverse(Array)
Array = Incrementing For loop
Array = Reverse(Array)
```

A reverse function for each array may be needed. Example of a reverse function:
```
const SIZE= uint32_t:8;

fn reverse_array(array: int64_t[SIZE]) ->
int64_t[STREAM_LENGTH_SIZE] {
for (i, new_array): (uint32_t, int64_t[SIZE])
in range(uint32_t:0, SIZE/uint32_t:2) {
let first: int64_t = array[i];
let second: int64_t = array[SIZE- i - uint32_t:1];
let array = update(array, SIZE- i - uint32_t:1, first);
update(array, i, second)
}(array)
}
```

There are two solutions. A reverse function in std lib or enable decrementing traversal.

Related to https://github.com/google/xls/issues/634, the DSLX loops must start from 0 and increment.