media/gpu: Optimize MT2T->AR30 shader

Optimize merging LSBs with MSBs for the MT2T->AR30 shader.

Before: 233 FPS
After: 269 FPS

Bug: b:304781371
Test: Tested by running VulkanImageProcessorPerfTest.Detile,
ImageProcessorBackendTest.VulkanDetileScaleTest, and
ImageProcessorBackendTest.VulkanMT2TDetileScaleTest.

Change-Id: I65627e69e1e673304e187aeb3f995f31d2a5317e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5414095


Commit-Queue: Justin Green <greenjustin@google.com>
Reviewed-by: Pilar Molina Lopez <pmolinalopez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282035}