i965/vec4: Trivial improvements to the with_writemask() function.

i965/vec4: Trivial improvements to the with_writemask() function.

Add assertion that the register is not in the HW_REG or IMM file,
calculate the conjunction of the old and new mask instead of replacing
the old [consistent with the behavior of brw_writemask(), causes no
functional changes right now], make it static inline to let the
compiler do a slightly better job at optimizing things, and shorten
its name.

v2: Assert that the new writemask is not zero to avoid undefined
    hardware behaviour.

Reviewed-by: Paul Berry <stereotype441@gmail.com>