If the clear is not mid render pass, clearWithLoadOp was used to either:

- modify the current render pass loadOps, assuming no rendering has been
  done, or
- defer the clears by staging them in the attachment images.

The former path however is dead code.  It's impossible to start the
render pass without recording any commands.  In other words, if the
render pass has already started, the clear must be mid RP.

Bug: angleproject:4836
Change-Id: Idb1cb37b8a0e56b897ac69cf435f9a52be4bd2f4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2473764Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>