Only use the upper 8 bits of the 16 bit fields when reading
and outputting 8 bit values in order to get exact
representations of x/255 for each possible 8 bit value.

Bug: swiftshader:122
Bug: b/129523279

Change-Id: I0cd54eb325c24f2fc5a53f12a0b90696024ca9d1
Tests: dEQP-VK.memory.pipeline_barrier.all.*
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33028Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>