intel/compiler/fs: Set up subgroup invocation as a system value

Subgroup invocation is computed using a vector immediate and some
dispatch-aware arithmetic.  Unfortunately, due to the vector arithmetic,
and the fact that it's frequently read 16-wide, it's not something that
can easily be CSEd by the back-end compiler.  There are a few different
possible approaches to this problem:

 1) Emit the code to calculate the subgroup invocation on-the-fly and
    trust NIR to do the CSE.  This is what we were doing.

 2) Add a back-end instruction for the subgroup ID.  This has the
    advantage of helping the back-end compiler with CSE but has the
    downside of very poor scheduling for the calculation because it has
    to be emitted in the back-end.

 3) Emit the calculation at the top of the program and re-use the
    result.  This gets rid of the CSE problem but comes at the cost of
    an extra live register.

This commit switches us from 1) to 3).  We choose to store the subgroup
invocation values as a W type to reduce the impact of the extra live
register.  Trusting NIR and using 1) was fine but we're soon going to
want to use the subgroup invocation value for other things in the
back-end compiler and this makes it much easier to do without having to
worry about CSE problems.

Reviewed-by: Iago Toral Quiroga <[email protected]>

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