st/glsl_to_nir: remove dead local variables

st/glsl_to_nir: remove dead local variables

Without this we do not end up with a deterministic NIR because
temporary register variables are added in random order. NIR must
be deterministic because we use it to produce a sha for the
radeonsi backends disk cache.

This fixes the shader cache for a bunch of shaders.

Another positive is that this results in a large reduction in the
size of the NIR that the state tracker stores to the disk cache.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>