i965/fs: Pull ir_binop_min/ir_binop_max handling to a separate function.

i965/fs: Pull ir_binop_min/ir_binop_max handling to a separate function.

This will be reused from the ARB_fp compiler.  I touched up the pre-gen6 path
to not overwrite dst in the first instruction, which prevents the need for
aliasing checks (we'll need that in the ARB_fp compiler, but it actually
hasn't been needed in this codebase since the revert of the nasty old
MOV-avoidance code).  I also made the conditional_mod between gen6 and
pre-gen6 consistent, which shouldn't matter except for denorm/(+/-)0
comparisons where the choice between left and right hand side of the
comparison changes.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>