i965: Lower bitfieldInsert.

i965: Lower bitfieldInsert.

v2: Only lower bitfieldInsert to BFM+BFI (and don't lower
    bitfieldExtract at all) since three-source instructions are now
    usable in the vertex shader.
v3: Lower bitfield_insert in the same pass with everything else, since
    it doesn't produce any instructions to be lowered (the other two
    lowering passes that were in a previous iteration of this series
    emitted subtractions which needed to be lowered).

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz> [v2]