i965/fs: Fix extract_i8/u8 to a 64-bit destination

The MOV instruction can extract bytes to words/double words, and
words/double words to quadwords, but not byte to quadwords.

For unsigned byte to quadword, we can read them as words and AND off the
high byte and extract to quadword in one instruction. For signed bytes,
we need to first sign extend to word and the sign extend that word to a
quadword.

Fixes the following test on CHV, BXT, and GLK:
   KHR-GL46.shader_ballot_tests.ShaderBallotBitmasks
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103628


Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

(cherry picked from commit 6ac2d169)