[ir] Fix DCE for multiple unused variables

It was modifying the instruction list while iterating over
it. Instead, keep track of the next instruction in the list and use a
while loop.

Change-Id: Iaaf0b498480e625511ddc53f9e312c06dac81b62
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/257014
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>