i965/vs: Add support for simple algebraic optimizations.

i965/vs: Add support for simple algebraic optimizations.

We generate silly code for array access, and it's easier to generally
support the cleanup than to specifically avoid the bad code in each
place we might generate it.

Removes 4.6% of instructions from 41.6% of shaders in shader-db,
particularly savage2/hon and unigine.

v2: Fixes by Ken: Make is_zero/one member functions, and fix a
    progress flag.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>