vc4: Add a "qir_for_each_inst_inorder" macro and use it in many places.

vc4: Add a "qir_for_each_inst_inorder" macro and use it in many places.

We have the prior list_foreach() all over the code, but I need to move
where instructions live as part of adding support for control flow.  Start
by just converting to a helper iterator macro.  (The simpler
"qir_for_each_inst()" will be used for the for-each-inst-in-a-block
iterator macro later)