All child nodes of selection should be sequence nodes, so that they will
output braces and extra braces can be removed from HLSL output.

Also make RewriteElseBlocks to reuse common IntermTraverser functionality
to simplify the code.

TEST=WebGL conformance tests on D3D9
BUG=angleproject:1013

Change-Id: Iafdc05468b22d110abcd020cf52c646dd98fb4a0
Reviewed-on: https://chromium-review.googlesource.com/273608Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>