openfimg: compiler: Fix wrong swizzle calculation

Apparently 'w' is not the next letter in the alphabet after 'z'. Too
much staring at vectors...

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Fix handling of operations without dst in liveness

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: compiler: Silence the noisy register allocator

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