intel/fs: Promote execution type to 32-bit when any half-float conversion is needed.

intel/fs: Promote execution type to 32-bit when any half-float conversion is needed.

The docs are fairly incomplete and inconsistent about it, but this
seems to be the reason why half-float destinations are required to be
DWORD-aligned on BDW+ projects.  This way the regioning lowering pass
will make sure that the destination components of W to HF and HF to W
conversions are aligned like the corresponding conversion operation
with 32-bit execution data type.

Tested-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>