PromoteInitializersToConstVar was erroring on for loops that contained array or structure constructor expressions.
Added lots more tests.
Fixed: tint:1364
Change-Id: I033eaad94756ea496fc8bc5f03f39c6dba4e3a88
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/75580
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>