openfimg: compiler: Move temporary and predicate allocation helpers

They belong to code generator because it has the knowledge about
registers/variables used by TGSI source.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Remove some dead code

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Avoid repeat/depart nodes as a part of statement list

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Print empty statements lists

Special "Nothing" keyword is used to resemble CORTL.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Add AST clean-up pass

A clean-up pass is added to remove empty lists from AST and explicitly
depart regions that do not have depart or repeat nodes yet.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>