i965/vec4: Eliminate dead writes to the flag register.

i965/vec4: Eliminate dead writes to the flag register.

For each write, search previous instructions for unread writes to the
flag register and remove them. Note that this will not eliminate the
last unread write.

total instructions in shared programs: 788074 -> 788004 (-0.01%)
instructions in affected programs:     4930 -> 4860 (-1.42%)

Reviewed-by: Eric Anholt <eric@anholt.net>