i965/fs: Explicitly set the exec_size on the add(32) in interpolation setup

i965/fs: Explicitly set the exec_size on the add(32) in interpolation setup

Soon we will start using the builder to explicitly set all the execution
sizes.  We could make a 32-wide builder, but the builder asserts that we
never grow it which is usually a reasonable assumption.  Since this one
instruction is a bit of an odd-ball, we just set the exec_size explicitly.

v2: Explicitly new the fs_inst instead of using the builder and setting
    exec_size after the fact.

v3: Set force_writemask_all with the builder instead of directly.  The
    builder over-writes it if we set it manually.  Also, if we don't have
    force_writemask_all in the builder it will assert-fail on SIMD32.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Acked-by: Francisco Jerez <currojerez@riseup.net>