glsl: Fix loop analysis of nested loops.

Previously, when visiting a variable dereference, loop analysis would
only consider its effect on the innermost enclosing loop.  As a
result, when encountering a loop like this:

    for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
        ...
        i = 2;
      }
    }

it would incorrectly conclude that the outer loop ran three times.

Fixes piglit test "vs-inner-loop-modifies-outer-loop-var.shader_test".

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

src/glsl/loop_analysis.h

@@ -167,8 +167,11 @@ public:
    /** Are all variables in the RHS of the assignment loop constants? */
    bool rhs_clean;
 
-   /** Is there an assignment to the variable that is conditional? */
-   bool conditional_assignment;
+   /**
+    * Is there an assignment to the variable that is conditional, or inside a
+    * nested loop?
+    */
+   bool conditional_or_nested_assignment;
 
    /** Reference to the first assignment to the variable in the loop body. */
    ir_assignment *first_assignment;
@@ -197,7 +200,7 @@ public:
    {
       const bool is_const = (this->num_assignments == 0)
 	 || ((this->num_assignments == 1)
-	     && !this->conditional_assignment
+	     && !this->conditional_or_nested_assignment
 	     && !this->read_before_write
 	     && this->rhs_clean);
 
@@ -214,7 +217,8 @@ public:
       return is_const;
    }
 
-   void record_reference(bool in_assignee, bool in_conditional_code,
+   void record_reference(bool in_assignee,
+                         bool in_conditional_code_or_nested_loop,
                          ir_assignment *current_assignment);
 };