These are local copies of source seen earlier in the trace.
Centralize them so they can be tracked across contexts.

Test: PUBG MEC
Bug: b/165824228
Bug: angleproject:4048
Change-Id: If414826b0280c61507812c2fd92706b7f095cc4b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2462162
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>