For struct varyings with nested structs, we would count registers
incorrectly, and produce D3D link errors.

This fixes tests in the dEQP's shaders.linkage.varying.struct.

BUG=angle:910

Change-Id: I47ea6dba36bf57cf94a7e7f30997c6c9096c2b40
Reviewed-on: https://chromium-review.googlesource.com/247243Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>