openfimg: compiler: Beautify dumps of trivial AST nodes

Empty repeat and depart nodes are quite usual.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Set dst mask to x___ in if's setp_eq

Other channels are not used by TGSI instruction, so generate code
accordingly.

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