pan/midgard: Add mir_insert_instruction*scheduled helpers

pan/midgard: Add mir_insert_instruction*scheduled helpers

In order to run register allocation after scheduling, it is sometimes
necessary to be able to insert instructions into an already-scheduled
program. This is suboptimal, since it forces us to do a worst-case
scheduling, but it is nevertheless required for correct handling of
spills/fills. Let's add helpers to insert instructions as standalone
bundles for use in spilling code.

These helpers are minimal -- they *only* work on load/store ops or
moves. They should not be used for anything but register spilling; any
other instructions should be added prior to the schedule.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>