i965: Don't use brw_set_conditionalmod in the FS and vec4 compilers.

i965: Don't use brw_set_conditionalmod in the FS and vec4 compilers.

brw_set_conditionalmod and brw_next_insn work together to set the
conditional modifier for the next instruction, then turn it off.
The Gen8+ generators don't implement this: we just set it for all future
instructions, and whack it for each fs_inst/vec4_instruction.

Both approaches work out because we only set conditional_mod on
IR instructions like CMP, AND, and so on, which correspond to exactly
one assembly instruction.  The Gen8 generators would break if we had
an IR instruction that generated multiple instructions, and the Gen4-7
EU emit layer would do...something.

To safeguard against this, assert that we only generated one instruction
if conditional_mod is set, and just set the flag directly on that
instruction rather than altering default state.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>