tgsi: add missing functionality to support instructions with labels

tgsi: add missing functionality to support instructions with labels

Could previously emit opcodes with label arguments, but was no way to
patch them with the actual destinations of those labels.

Adds two functions:

  ureg_get_instruction_number - to get the id of the next instruction
     to be emitted

  ureg_fixup_label - to patch an emitted label to point to a given
     instruction number.

Need some more complex examples than u_simple_shader, so far this has
only been compile-tested.