Make sure that only one sequence operator is split on one iteration
of SplitSequenceOperator. This prevents multiple successive PostVisit
calls to nested sequence operator nodes from adding duplicate nodes
to the AST. The sequence operators are split starting from the
outermost one to preserve execution order.

Note that the shader translator somewhat unexpectedly generates nested
sequence operators in the AST when there is a sequence operator with
more than two operands, so this bug ended up affecting shaders in the
wild. The code around parsing sequence operators could be clarified
separately.

BUG=638313
TEST=angle_end2end_tests

Change-Id: Ic6400a484ceff0c790c2290f7b4b80980f87cd88
Reviewed-on: https://chromium-review.googlesource.com/376678Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>