Transforms a for-loop into a loop.
Will be required by the SPIR-V writer.
Bug: tint:952
Change-Id: Iba859bd144d702cee85374f2cfcb94cd7465ca98
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57202
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>