i965: Add support for the DP2 opcode, which we use for dot(vec2, vec2).

i965: Add support for the DP2 opcode, which we use for dot(vec2, vec2).

The original glsl compiler would generate a.x * b.x + a.y * b.y, which
we would do mul+mul+add for instead of this mul+mac.

Fixes glsl-fs-dot-vec2.