i965: Disable try_emit_b2f_of_compare on Gen4-6.

The optimization relies on CMP setting the destination to 0, which is
equivalent to 0.0f.  However, early platforms only set the least
significant byte, leaving the other bits undefined.  So, we must disable
the optimization on those platforms.

Oddly, Sandybridge wasn't reported as broken.  The PRM states that it
only sets the LSB, but the internal documentation says that it follows
the IVB behavior.  Since it wasn't reported as broken, we believe it
really does follow the IVB behavior.

v2: Allow the optimization on Sandybridge (requested by Matt).

+32 piglits on Ironlake.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?=79963
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chris Forbes <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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