i965: Fix hangs with FP KIL instructions pre-gen6.
i965: Fix hangs with FP KIL instructions pre-gen6.

We can't support IF statements in 16-wide on these.  To get back to 16-wide
for these shaders, we need to support predicate on discard instructions in the
backend IR, which is something we've sort of got on the list to do anyway.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55828
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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