ProgramExecutable::isCompute() is being updated to no longer use
mProgramState or mProgramPipelineState, as part of the effort to remove
those members from ProgramExecutable. Functionally, things are the same
for PPOs, but Programs are being updated to use the boolean
ProgramExecutable::mIsCompute rather than checking the linked shader
stages to determine draw vs compute.

Slightly unrelated, but this CL also removes the unused function
ProgramPipeline::hasImages() which was missed in one of the earlier CLs
in this relation chain.

Bug: angleproject:4520
Test: Build/CQ
Change-Id: Ief28021310d6d0b1be5b7608a59deb87b0cf591d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2243326
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>