nir/spirv: Handle builtins in OpAccessChain

nir/spirv: Handle builtins in OpAccessChain

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.