Geometry shaders weren't being processed in some of the separable
shader logic. This CL fixes two cases: one in Program and one in
ProgramPipeline.

Bug: angleproject:5409
Change-Id: I19adc5c11a54814d28dd20574a5e038ca9dbd021
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2574827
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>