Remove if-break deprecation

This CL removes support or if-break and requires the use of break-if.

Bug: tint:1724
Change-Id: I8311de2f0ce11b5af7fada71d258ae441f9e42f8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/111100
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

test/tint/vk-gl-cts/graphicsfuzz/discard-in-loop-in-function/0-opt.wgsl

@@ -22,10 +22,7 @@ fn f_() {
     discard;
 
     continuing {
-      if (false) {
-      } else {
-        break;
-      }
+      break if !(false);
     }
   }
   return;