i965/fs: Jump from discard statements to the end of the program when done.

i965/fs: Jump from discard statements to the end of the program when done.

From the GLSL 1.30 spec:

     The discard keyword is only allowed within fragment shaders. It
     can be used within a fragment shader to abandon the operation on
     the current fragment. This keyword causes the fragment to be
     discarded and no updates to any buffers will occur. Control flow
     exits the shader, and subsequent implicit or explicit derivatives
     are undefined when this control flow is non-uniform (meaning
     different fragments within the primitive take different control
     paths).

v2: Don't emit the final HALT if no other HALTs were emitted.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)