i965/fs: Add basic-block-level dead code elimination.

i965/fs: Add basic-block-level dead code elimination.

This is a poor substitute for proper global dead code elimination that
could replace both our current paths, but it was very easy to write.  It
particularly helps with Valve's shaders that are translated out of DX
assembly, which has been register allocated and thus have a bunch of
unrelated uses of the same variable (some of which get copy-propagated
from and then left for dead).

shader-db results:
total instructions in shared programs: 1735753 -> 1731698 (-0.23%)
instructions in affected programs:     492620 -> 488565 (-0.82%)

v2: Fix comment typo

Reviewed-by: Matt Turner <mattst88@gmail.com>