i965: Correctly emit the RNDZ instruction.

Simply using RNDU, RNDZ, or RNDE does not produce the desired result.
Rather, the RND* instructions place a value in the destination register
that may be 1 less than the correct answer.  They can also set per-channel
"increment bits" in a flag register, which, if set, mean dest needs to
be incremented by 1.  A second instruction - a predicated add -
completes the job.

Notably, RNDD always produces the correct answer in a single
instruction.

Fixes piglit test glsl-fs-trunc.

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