i915: link errors on alu/tex limits, and group loads for tex limits.

In xorg/xserver!1171 (closed) it would be nice if we could just detect at compile time if the shaders would definitely not run on i915. This series does a test compile at link time (even if we throw out the i915 code and regenerate it later), and returns a useful error message from glLinkShader()'s status. And, once I had a clear signal about some of these limits, we can throw in nir_group_loads() to greatly reduce how often we hit limits across shader-db.