i965/fs: Create and emit instructions in one step in opt_peephole_sel.

i965/fs: Create and emit instructions in one step in opt_peephole_sel.

This simplifies opt_peephole_sel() slightly by emitting the SEL
instructions immediately after they are created, what makes the
sel_inst and mov_imm_inst arrays unnecessary and will make it possible
to get rid of the explicit inserts when the pass is migrated to the IR
builder.

Reviewed-by: Matt Turner <mattst88@gmail.com>