i965/fs: don't make unused payload registers interfere

i965/fs: don't make unused payload registers interfere

Before, we were setting payload_last_use_ip for unused payload
registers to 0, which made them interfere with whatever the first
instruction wrote to due to the workaround for SIMD16 uniform arguments.
Just use -1 to mean "unused" instead, and then skip setting any
interferences for unused payload registers.

instructions in affected programs:     0 -> 0
helped:                                0
HURT:                                  0
GAINED:                                1
LOST:                                  0

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Connor Abbott <connor.w.abbott@intel.com>