intel/fs: Add a SCRATCH_HEADER opcode

intel/fs: Add a SCRATCH_HEADER opcode

This opcode is responsible for setting up the buffer base address and
per-thread scratch space fields of a scratch message header.  For the
most part, it's a copy of g0 but some messages need us to zero out g0.2
and the bottom bits of g0.5.

This may actually fix a bug when nir_load/store_scratch is used.  The
docs say that the DWORD scattered messages respect the per-thread
scratch size specified in gN.3[3:0] in the message header but we've been
leaving it zero.  This may mean that we've been ignoring any scratch
reads/writes from a load/store_scratch intrinsic above the 1KB mark.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7084>