Loops are similar to if-else in that they generate a number of blocks
where the first block specifies divergence (OpLoopMerge) and the merge
block.  Differently from if-else, there is a block where the condition
is evaluated and a block which `continue;` leads to (this last block is
the only one allowed to back-jump to the beginning of the loop).

Bug: angleproject:4889
Change-Id: Ic59f4bf3e05fbf93cb5af85acd3bc4b0da8412af
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2957809
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>