The code was using the LSB instead of the MSB.
Added shifts to fix the issue.

Change-Id: I6bdfc367c6daea98b0dd8878ea9c0076574206f5
Tests: dEQP-VK.renderpass.suballocation.multisample.a2b10g10r10_unorm_pack32.samples_4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32789
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>