i965 new VS: Fix copy propagation of double negatives.

i965 new VS: Fix copy propagation of double negatives.

When copy propagating a value into an instruction that negates its
argument, we need to invert the sense of the value's "negate" flag, so
that -(+x) becomes -x and -(-x) becomes +x.

Previously, we were always setting the value's "negate" flag to true
in this circumstance, so that both -(+x) and -(-x) turned into -x.

Fixes Piglit test vs-double-negative.shader_test.

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