intel/compiler: generalize the combine constants pass

intel/compiler: generalize the combine constants pass

At the very least we need it to handle HF too, since we are doing
constant propagation for MAD and LRP, which relies on this pass
to promote the immediates to GRF in the end, but ideally
we want it to support even more types so we can take advantage
of it to improve register pressure in some scenarios.

v2 (Jason):
 - Support 64-bit types too.
 - Check if we need to set the half-float flag if the immediate already
   existed.
 - Multiply the size of the immediate by the width of the copy

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>