i965/fs: Add (sub)reg_offset asserts to brw_reg_from_fs_reg.
i965/fs: Add (sub)reg_offset asserts to brw_reg_from_fs_reg.

These are completely ignored by the conversion to brw_reg, so they
better be zero.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler