i965/vec4: Introduce VEC4 IR builder.

i965/vec4: Introduce VEC4 IR builder.

See "i965/fs: Introduce FS IR builder." for the rationale.

v2: Drop scalarizing VEC4 builder.
v3: Take a backend_shader as constructor argument.  Improve handling
    of debug annotations and execution control flags.  Rename "instr"
    variable.  Initialize cursor to NULL by default and add method to
    explicitly point the builder at the end of the program.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>