With certain selection statements with a vertex input in the
condition and side-effects in the else-block, we'd run in to
a D3D9 compiler bug which would cause incorrect results.

We can work around this bug in D3D9 by selectively rewriting
these statements to use an 'else if' clause instead of 'else'.

BUG=322794

Change-Id: I93c96fb201ff4959c00d9a36321faac7e0343278
Reviewed-on: https://chromium-review.googlesource.com/184681Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>