Split sequence operators if some of their operands generate statements
in subsequent AST transformations to guarantee the right order of
execution. For now, this is supported for expressions that return
arrays and unfolded short-circuiting operators, which is enough to get
WebGL 2 tests passing. A trickier corner case with dynamic indexing of
vectors as an l-value is left to be addressed later.

BUG=angleproject:1341
TEST=angle_end2end_tests, WebGL 2 conformance test:
     conformance2/glsl3/array-in-complex-expression.html

Change-Id: I9301edd3366be7607a8aa4c42a5ec13928749e10
Reviewed-on: https://chromium-review.googlesource.com/361694Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>