mesa/st/glsl_to_tgsi: Add tracking of ifelse writes in register merging

mesa/st/glsl_to_tgsi: Add tracking of ifelse writes in register merging

Improve the life-time evaluation of temporary registers by also tracking
writes in both if and else branches and in up to 32 nested scopes.
As a result the estimated required register life-times can be further
reduced enabling more registers to be merged.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>