SPV 1.4: Implement the 5 new loop controls.

Test/spv.1.4.LoopControl.frag

@@ -0,0 +1,19 @@
+#version 450
+
+#extension GL_EXT_control_flow_attributes : enable
+
+bool cond;
+
+void main()
+{
+        [[min_iterations(3), max_iterations(7)]]   for (int i = 0; i < 8; ++i) { }
+        [[iteration_multiple(2)]]                  while(true) {  }
+        [[peel_count(5)]]                          do {  } while(true);
+        [[partial_count(4)]]                       for (int i = 0; i < 8; ++i) { }
+
+        // warnings on all these
+        [[min_iterations, max_iterations]]   for (int i = 0; i < 8; ++i) { }
+        //[[iteration_multiple(0)]]                  while(true) {  }
+        //[[peel_count]]                          do {  } while(true);
+        //[[partial_count]]                       for (int i = 0; i < 8; ++i) { }
+}