Scalarizing vec and mat constructor args can generate new statements
in the parent block of the constructor. To preserve the correct
execution order of expressions, scalarized vector and matrix
constructors need to be first moved out from inside loop conditions
and sequence operators. This is done whenever the compiler flag to
scalarize args is on.

BUG=chromium:772653
TEST=angle_unittests

Change-Id: Id40f8d848a9d087e186ef2e680c8e4cd440221d9
Reviewed-on: https://chromium-review.googlesource.com/790412
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>