RewriteElseBlocks used to have an issue where it could add an
unprefixed struct name to the AST in a TIntermRaw node, as opposed to
the prefixed name that the struct would be defined with. Use a proper
return statement node instead of a raw node to fix this issue and make
the code more robust.

BUG=angleproject:2061
TEST=angle_unittests

Change-Id: I3993b5093646983f038268f3a5ffe26ccdae66e8
Reviewed-on: https://chromium-review.googlesource.com/530785Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>