Vulkan: SPIR-V Gen: Support scalar constructors In GLSL, the scalar constructor can be given a vector and matrix, in which case the first element is selected. This case was not previously handled. Bug: angleproject:4889 Change-Id: Ibf276883fc7396b750981a4e469ff9b152c6e700 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3042554 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>