i965: Fix warnings introduced in commit e24ef5ab.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
i965/vec4: Add basic common subexpression elimination.

[mattst88]: Modified to perform CSE on instructions with
            the same writemask. Offered no improvement before.

total instructions in shared programs: 1995633 -> 1995185 (-0.02%)
instructions in affected programs:     14410 -> 13962 (-3.11%)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
i965/vec4: Improve CSE performance by expiring some available expressions.

Port of commit 5daf867f to the vec4 code.
i965/vec4: Don't emit null MOVs in CSE.

Port of commit 219b43c6 to the vec4 code.
i965/vec4: Perform CSE on CMP(N) instructions.

Port of commit b16b3c87 to the vec4 code.

No shader-db improvements, but might as well. The fs backend saw an
improvement because it's scalar and multiple identical CMP instructions
were generated by the SEL peepholes.
i965/fs: Disable unlit_centroid_workaround on Haswell.

Although the HSW PRM shows it, the BSpec lists this workaround as being
for Ivybridge only.

total instructions in shared programs: 1994951 -> 1993675 (-0.06%)
instructions in affected programs:     27325 -> 26049 (-4.67%)
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