If the initializer is a constant, it's specified directly in the
OpVariable instruction.  Otherwise an OpStore is generated where the
declaration is visited.

Bug: angleproject:4889
Change-Id: I79291552ecc50c0878f921aff9f6d8618be34116
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939331
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>