i965/fs: Calculate the (un)spill block size correctly.

i965/fs: Calculate the (un)spill block size correctly.

Currently the spilling code attempts to guess the scratch message
block size from the dispatch width of the shader, which is plain wrong
for SIMD-lowered instructions (frequently but not exclusively
encountered in SIMD32 shaders) or for instructions with register
region data types of size other than 32 bit.

Instead try to use the SIMD component size of the instruction which in
some cases will allow the dataport to apply the correct channel mask
to the scratch data read or written.  In the spill case the block size
needs to be clamped to the number of MRF registers reserved for
spilling.  In the unspill case I didn't even bother because we
currently have no 100% accurate way to determine whether a source
region is per-channel or whether it contains things like headers that
don't respect channel boundaries -- That's fine, because the unspill
is marked force_writemask_all we can just use the largest allowable
scratch message size.

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