intel/compiler: Add a "base class" for program keys

Right now, all keys have two things in common: a program string ID and a
sampler_prog_key_data.  I'd like to add another thing or two and need a
place to put it.  This commit adds a new brw_base_prog_key struct which
contains those two common bits.

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