glsl/loops: Move some analysis from loop_controls to loop_analysis.

glsl/loops: Move some analysis from loop_controls to loop_analysis.

Previously, the sole responsibility of loop_analysis was to find all
the variables referenced in the loop that are either loop constant or
induction variables, and find all of the simple if statements that
might terminate the loop.  The remainder of the analysis necessary to
determine how many times a loop executed was performed by
loop_controls.

This patch makes loop_analysis also responsible for determining the
number of iterations after which each loop terminator will terminate
the loop, and for figuring out which terminator will terminate the
loop first (I'm calling this the "limiting terminator").

This will allow loop unrolling to make use of information that was
previously only visible from loop_controls, namely the identity of the
limiting terminator.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>