i965/gen7: Set src/dst types for 3-src instructions.

i965/gen7: Set src/dst types for 3-src instructions.

Also update asserts to allow BFE and BFI2, which take (unsigned)
doubleword arguments.

v2: Allow BRW_REGISTER_TYPE_UD for src1 and src2 as well.
    Assert that src2.type (instead of src0.type) matches dest.type since
    it's the primary argument and src0 and src1 might correctly have
    different types.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz> [v1]