i965/fs: Reimplement dead_code_elimination().

i965/fs: Reimplement dead_code_elimination().

total instructions in shared programs: 1653399 -> 1651790 (-0.10%)
instructions in affected programs:     92157 -> 90548 (-1.75%)
GAINED:                                2
LOST:                                  2

Also significantly reduces the number of optimization loop iterations:

total loop iterations in shared programs: 39724 -> 31651 (-20.32%)
loop iterations in affected programs:     21617 -> 13544 (-37.35%)

Including some great pathological cases, like 29 -> 3 in Strike Suit
Zero and 24 -> 3 in Dota2.

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