glsl/loops: replace loop controls with a normative bound.

This patch replaces the ir_loop fields "from", "to", "increment",
"counter", and "cmp" with a single integer ("normative_bound") that
serves the same purpose.

I've used the name "normative_bound" to emphasize the fact that the
back-end is required to emit code to prevent the loop from running
more than normative_bound times.  (By contrast, an "informative" bound
would be a bound that is informational only).

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/loop_analysis.h

@@ -44,8 +44,7 @@ analyze_loop_variables(exec_list *instructions);
  *
  *  (if (expression bool ...) (break))
  *
- * and fill in the \c ir_loop::from, \c ir_loop::to, and \c ir_loop::counter
- * fields of the \c ir_loop.
+ * and fill in the \c normative_bound field of the \c ir_loop.
  *
  * In this process, some conditional break-statements may be eliminated
  * altogether.  For example, if it is provable that one loop exit condition will