Convert computeboids to for().

This CL updates computeboids to use the `for()` sugar instead of the
loop/continuing block.

Change-Id: I13f416624821a87fbfa40b15669980bedc794ed4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28842
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/test/compute_boids.wgsl b/test/compute_boids.wgsl
index 2ddcae1..bf4ce44 100644
--- a/test/compute_boids.wgsl
+++ b/test/compute_boids.wgsl
@@ -83,11 +83,7 @@
 
   var pos : vec2<f32>;
   var vel : vec2<f32>;
-  var i : u32 = 0u;
-  loop {
-    if (i >= 5u) {
-      break;
-    }
+  for(var i : u32 = 0u; i < 5u; i = i + 1) {
     if (i == index) {
       continue;
     }
@@ -106,10 +102,6 @@
       cVel = cVel + vel;
       cVelCount = cVelCount + 1;
     }
-
-    continuing {
-      i = i + 1u;
-    }
   }
   if (cMassCount > 0) {
     cMass =