This completely reworks the pass to support deref instructions and
delete support for old deref chains

Now that it's rewritten for deref instructions, we can turn it back on.

This should make nir_lower_tex properly handle deref instructions as
well as make it more correct when texture arrays are used and it's
called after lowering samplers to binding table indices.

It's only used by the ir3 stand-alone compiler and Rob said we could
delete it.

This deletes support for _var intrinsics and legacy deref chains in
favor of deref instructions.  The internals are also reworked a bit to
use deref instructions directly.

No one is currently using so we can make this change irrespective of
driver.  We may use it again in i965 so it's best to pretend to keep it
working.

This commit reworks nir_lower_vars_to_ssa to use deref instructions and
deref paths internally instead of deref chains.  We also drop support
for the old load/store/copy_var intrinsics.

This makes us build the is_direct parameter as the nodes are constructed
rather than as we walk the chain.  This will be useful later.

We change glsl_to_nir to provide derefs for bot textures and samplers
while we're at it.  This makes the lowering much easier since we only
either replace sources or remove them.

This little refactor makes us stop passing stage around and puts the
builder as the first parameter to some functions.

nir_tex_instr_create uses rzalloc so it's already NULL

Everything else should already be handled.