Previously we replicated the high bits into the low
and from that point on treated these values as 16 bits.

Unfortunately, that causes a snorm channel value of 0xff
to yield -1/32767 rather than the expected -1/127.

Bug: b/129523279
Test: dEQP-VK.image.mutable.*snorm*
Change-Id: I6e329490f92562d6edac86043e80fef978682c88
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31728Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>