We can check the RenderPass CB helper directly instead of caching a
separate variable.

Bug: angleproject:4911
Change-Id: Ic953c101c64fd5142e2dedfd06e3fea287331fd4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2341761Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>