Vectors or matrices that are dynamically indexed as a part of an
l-value generate new statements in the RemoveDynamicIndexing AST
transformation step. SplitSequenceOperator needs to detect this case
and split the sequence operator before statements are generated from
its operands to ensure the correct order of execution.

BUG=angleproject:1341
TEST=angle_end2end_tests

Change-Id: I84e41a59c88fb5d0111669cab60312b930531a22
Reviewed-on: https://chromium-review.googlesource.com/361695Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>