[tint] Fix continue bypassing decl check

We were missing cases where a reference to a bypassed variable was
inside another loop continuing block nested inside the outer loop's
continuing block. Use a loop to check all parent continuing blocks,
not just the first parent.

Bug: 353034820
Change-Id: Iffd4be81043f43f07aa39f87cd2d7f7fa4936c7f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/199455
Commit-Queue: David Neto <dneto@google.com>
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: David Neto <dneto@google.com>
(cherry picked from commit da0ac02382cea2820e23f69be709c54e43900999)
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/200056
Reviewed-by: Antonio Maiorano <amaiorano@google.com>