Range Analysis: Support step > 1 in for-loops when index is increasing

This patch implements the computations of the range on the
loop control variable which is increasing with a constant
integer step that is greater than 1.

For example:

// Range of i is [2u, 6u]
for (var i = 2u; i <= 6u; i += 2u) {
  // ...
}

// Range of i is [2, 8]
for (var i = 2; i <= 10; i += 3) {
  // ...
}

Bug: 348701956
Change-Id: I0a7d0b837718002a42419481d69b82c54448ee91
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/337495
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Shao, Jiawei <jiawei.shao@intel.com>