mesa/st/glsl_to_tgsi: Properly resolve life times simple if/else + use constructs

mesa/st/glsl_to_tgsi: Properly resolve life times simple if/else + use constructs

in constructs like below, currently the live range estimation extends the live range
of t unecessarily to the whole loop because it was not detected that t is
unconditional written and later read only in the "if (a)" scope.

  while (foo)  {
    ...
    if (a) {
       ...
       if (b)
         t = ...
       else
         t = ...
       x = t;
       ...
    }
     ...
  }

This patch adds a unit test for this case and corrects the minimal live range estimation
accordingly.

v4: update comments
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>