i965/fs: Fix a bug with dead_code_eliminate on large writes

i965/fs: Fix a bug with dead_code_eliminate on large writes

Previously, if an instruction wrote to more than one register, we
implicitly assumed that it filled the entire register.  We never hit this
before because the only time we did multi-register writes was things like
texturing which always wrote to all of the registers.  However, with the
upcoming ability to do 16-wide instructions in SIMD8 and things of that
nature, we can have multi-register writes at offsets and we'll hit this.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>