Previously, we were trying to handle them later when loading. However, at
that point, you've already lost information and it's harder to handle
certain corner-cases. In particular, if you have a shader that does
gl_PerVertex.gl_Position.x = foo
we have trouble because we see the .x and we don't know that we're in
gl_Position. If we, instead, handle it in OpAccessChain, we have all the
information we need and we can silently re-direct it to the appropreate
variable. This also lets us delete some code which is a nice side-effect.
Jason Ekstrand
7e6959402d