mesa/st/glsl_to_tgsi: Mark first write as unconditional when appropriate

mesa/st/glsl_to_tgsi: Mark first write as unconditional when appropriate

In the register lifetime estimation if the first write is unconditional or
conditional but not within a loop then this is an unconditional dominant
write in the sense of register life time estimation.
Add a test case and record the write accordingly.

Fixes: 807e2539e512ca6c96f059da855473eb7be99ba1 ("mesa/st/glsl_to_tgsi: Add
tracking of ifelse writes in register merging")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104803
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>