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>

src/tint/transform/remove_unreachable_statements_test.cc

@@ -209,9 +209,7 @@ fn f() {
   loop {
 
     continuing {
-      if (true) {
-        break;
-      }
+      break if true;
     }
   }
   var preserve_me = 1;