Vulkan: SPIR-V Gen: Take advantage of OpConstantNull As part of fixing GLSLTest*.MissingReturn* tests, refactoring is done such that complex null values are declared concisely with OpConstantNull. The tests are fixed by making the SPIR-V generator implicitly add a `return <null-value-of-correct-type>;` if the return statement is missing from GLSL. Bug: angleproject:4889 Change-Id: Iffd80c5e6ce66afcdbf3813bffade6928f292007 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3010578 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>