i965: Add trivial dead code elimination in the new FS backend.

i965: Add trivial dead code elimination in the new FS backend.

The glsl core should be handling most dead code issues for us, but we
generate some things in codegen that may not get used, like the 1/w
value or pixel deltas.  It seems a lot easier this way than trying to
work out up front whether we're going to use those values or not.