[uniformity] Process index before object

In an index accessor expression, the index expression needs to be
processed before the object expression to capture potential for the
index expression to modify the uniformity of the object itself (via a
function call).

Fixed: 505389751
Change-Id: I3f403232745d1f6c720b994a2a1d0dcbe9171562
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/306076
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Alan Baker <alanbaker@google.com>